diff --git a/apps/admin/src/pages/Classes/detail.tsx b/apps/admin/src/pages/Classes/detail.tsx
index 435b46b..d2fbaa7 100644
--- a/apps/admin/src/pages/Classes/detail.tsx
+++ b/apps/admin/src/pages/Classes/detail.tsx
@@ -19,6 +19,7 @@ interface ClassStudent {
   studentName: string;
   studentNo: string;
   joinDate: string;
+  leaveDate: string | null;
   status: string;
 }
 
@@ -305,6 +306,7 @@ const ClassDetailPage: React.FC = () => {
     { title: '姓名', dataIndex: 'studentName' },
     { title: '学号', dataIndex: 'studentNo' },
     { title: '加入日期', dataIndex: 'joinDate' },
+    { title: '离班日期', dataIndex: 'leaveDate', render: (v: string | null) => v || '-' },
     {
       title: '状态',
       dataIndex: 'status',
@@ -316,11 +318,12 @@ const ClassDetailPage: React.FC = () => {
     },
     {
       title: '操作',
-      render: (_: unknown, r: ClassStudent) => (
-        <Popconfirm title="确认移除？" onConfirm={() => handleRemoveStudent(r.studentId)}>
-          <PermissionButton permission="class:edit" size="small" danger>移除</PermissionButton>
-        </Popconfirm>
-      ),
+      render: (_: unknown, r: ClassStudent) =>
+        r.status === 'active' ? (
+          <Popconfirm title="确认移除？" onConfirm={() => handleRemoveStudent(r.studentId)}>
+            <PermissionButton permission="class:edit" size="small" danger>移除</PermissionButton>
+          </Popconfirm>
+        ) : null,
     },
   ];
 
diff --git a/apps/server/src/classes/classes.batch-import-membership.spec.ts b/apps/server/src/classes/classes.batch-import-membership.spec.ts
new file mode 100644
index 0000000..fea8627
--- /dev/null
+++ b/apps/server/src/classes/classes.batch-import-membership.spec.ts
@@ -0,0 +1,47 @@
+import { ClassesService } from './classes.service';
+import { ClassStudent } from '../entities';
+
+describe('ClassesService — DingTalk class import membership lifecycle', () => {
+  it('reactivates left memberships and skips active memberships', async () => {
+    const left = {
+      classId: 3,
+      studentId: 8,
+      status: 'left',
+      joinDate: '2026-01-01',
+      leaveDate: '2026-02-01',
+    } as ClassStudent;
+    const active = { classId: 3, studentId: 9, status: 'active' } as ClassStudent;
+    const classStudentRepo = {
+      find: jest.fn().mockResolvedValue([left, active]),
+      create: jest.fn().mockImplementation((value: Partial<ClassStudent>) => value),
+      save: jest.fn().mockImplementation(async (value: ClassStudent[]) => value),
+    };
+    const service = new ClassesService(
+      { findOne: jest.fn().mockResolvedValue({ id: 3 }) } as never,
+      classStudentRepo as never,
+      {} as never,
+      {} as never,
+      {} as never,
+      {} as never,
+      { create: jest.fn(), save: jest.fn() } as never,
+      {
+        find: jest.fn().mockResolvedValue([
+          { dingUserId: 'd8', studentId: 8 },
+          { dingUserId: 'd9', studentId: 9 },
+        ]),
+        create: jest.fn(),
+        save: jest.fn(),
+      } as never,
+    );
+
+    const result = await service.batchImportStudents(3, [
+      { dingUserId: 'd8', name: '学生8' },
+      { dingUserId: 'd9', name: '学生9' },
+    ]);
+
+    expect(result).toEqual({ imported: 1, skipped: 1 });
+    expect(left).toMatchObject({ status: 'active', leaveDate: null });
+    expect(left.joinDate).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    expect(classStudentRepo.save).toHaveBeenCalledWith([left]);
+  });
+});
diff --git a/apps/server/src/classes/classes.membership.spec.ts b/apps/server/src/classes/classes.membership.spec.ts
new file mode 100644
index 0000000..9dc2167
--- /dev/null
+++ b/apps/server/src/classes/classes.membership.spec.ts
@@ -0,0 +1,95 @@
+import { BadRequestException, NotFoundException } from '@nestjs/common';
+import { ClassesService } from './classes.service';
+import { ClassStudent } from '../entities';
+
+function createService(
+  classStudentRepo: Record<string, jest.Mock>,
+  classRepo: Record<string, jest.Mock> = { findOne: jest.fn().mockResolvedValue({ id: 3 }) },
+  studentRepo: Record<string, jest.Mock> = {
+    find: jest.fn().mockResolvedValue([{ id: 8 }, { id: 9 }, { id: 10 }]),
+  },
+) {
+  return new ClassesService(
+    classRepo as never,
+    classStudentRepo as never,
+    {} as never,
+    {} as never,
+    {} as never,
+    {} as never,
+    studentRepo as never,
+    {} as never,
+  );
+}
+
+describe('ClassesService — student membership lifecycle', () => {
+  it('marks an active membership as left instead of deleting it', async () => {
+    const membership = {
+      classId: 3,
+      studentId: 8,
+      status: 'active',
+      leaveDate: null,
+    } as ClassStudent;
+    const repo = {
+      findOne: jest.fn().mockResolvedValue(membership),
+      save: jest.fn().mockImplementation(async (value: ClassStudent) => value),
+    };
+    const service = createService(repo);
+
+    await service.removeStudent(3, 8);
+
+    expect(membership.status).toBe('left');
+    expect(membership.leaveDate).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    expect(repo.save).toHaveBeenCalledWith(membership);
+  });
+
+  it('rejects removing an already-left membership', async () => {
+    const repo = {
+      findOne: jest.fn().mockResolvedValue({ status: 'left' }),
+      save: jest.fn(),
+    };
+    const service = createService(repo);
+
+    await expect(service.removeStudent(3, 8)).rejects.toBeInstanceOf(BadRequestException);
+    expect(repo.save).not.toHaveBeenCalled();
+  });
+
+  it('rejects removing a student without a membership', async () => {
+    const repo = {
+      findOne: jest.fn().mockResolvedValue(null),
+      save: jest.fn(),
+    };
+    const service = createService(repo);
+
+    await expect(service.removeStudent(3, 8)).rejects.toBeInstanceOf(NotFoundException);
+  });
+
+  it('reactivates left memberships, creates new ones, and skips active ones', async () => {
+    const left = {
+      id: 1,
+      classId: 3,
+      studentId: 8,
+      status: 'left',
+      joinDate: '2026-01-01',
+      leaveDate: '2026-02-01',
+    } as ClassStudent;
+    const active = { id: 2, classId: 3, studentId: 9, status: 'active' } as ClassStudent;
+    const repo = {
+      find: jest.fn().mockResolvedValue([left, active]),
+      create: jest.fn().mockImplementation((value: Partial<ClassStudent>) => value),
+      save: jest.fn().mockImplementation(async (value: ClassStudent[]) => value),
+    };
+    const service = createService(repo);
+
+    const result = await service.addStudents(3, [8, 9, 10, 10]);
+
+    expect(result).toEqual({ added: 2, skipped: 1 });
+    expect(left).toMatchObject({ status: 'active', leaveDate: null });
+    expect(left.joinDate).toMatch(/^\d{4}-\d{2}-\d{2}$/);
+    expect(repo.create).toHaveBeenCalledWith(
+      expect.objectContaining({ classId: 3, studentId: 10, status: 'active' }),
+    );
+    expect(repo.save).toHaveBeenCalledWith(
+      expect.arrayContaining([left, expect.objectContaining({ studentId: 10 })]),
+    );
+  });
+});
diff --git a/apps/server/src/classes/classes.service.ts b/apps/server/src/classes/classes.service.ts
index d61a45f..36451a6 100644
--- a/apps/server/src/classes/classes.service.ts
+++ b/apps/server/src/classes/classes.service.ts
@@ -227,34 +227,45 @@ export class ClassesService {
     }
 
     // 3. Fetch existing class-student links in one query
-    const allStudentIds = Array.from(dingToStudentId.values());
-    const alreadyInClass = new Set<number>();
-    if (allStudentIds.length > 0) {
-      const existingClassStudents = await this.classStudentRepo.find({
-        where: { classId, studentId: In(allStudentIds) },
-      });
-      for (const cs of existingClassStudents) {
-        alreadyInClass.add(cs.studentId);
+    const allStudentIds = Array.from(new Set(dingToStudentId.values()));
+    const existingClassStudents =
+      allStudentIds.length > 0
+        ? await this.classStudentRepo.find({
+            where: { classId, studentId: In(allStudentIds) },
+          })
+        : [];
+    const existingByStudentId = new Map(
+      existingClassStudents.map((classStudent) => [classStudent.studentId, classStudent]),
+    );
+    const today = new Date().toISOString().slice(0, 10);
+    let skipped = 0;
+    const memberships = allStudentIds.flatMap((studentId) => {
+      const existing = existingByStudentId.get(studentId);
+      if (existing?.status === 'active') {
+        skipped++;
+        return [];
       }
-    }
-
-    // 4. Batch insert new class-student records
-    const newClassStudents = allStudentIds
-      .filter((sid) => !alreadyInClass.has(sid))
-      .map((studentId) =>
+      if (existing) {
+        existing.status = 'active';
+        existing.joinDate = today;
+        existing.leaveDate = null;
+        return [existing];
+      }
+      return [
         this.classStudentRepo.create({
           classId,
           studentId,
           status: 'active',
-          joinDate: new Date().toISOString().slice(0, 10),
+          joinDate: today,
         }),
-      );
+      ];
+    });
 
-    if (newClassStudents.length > 0) {
-      await this.classStudentRepo.save(newClassStudents);
+    if (memberships.length > 0) {
+      await this.classStudentRepo.save(memberships);
     }
 
-    return { imported: newClassStudents.length, skipped: alreadyInClass.size };
+    return { imported: memberships.length, skipped };
   }
   async update(id: number, dto: UpdateClassDto) {
     const cls = await this.classRepo.findOne({ where: { id } });
@@ -315,26 +326,61 @@ export class ClassesService {
   }
 
   async addStudents(classId: number, studentIds: number[]) {
+    const uniqueStudentIds = [...new Set(studentIds)];
+    if (uniqueStudentIds.length === 0) return { added: 0, skipped: 0 };
+
+    const cls = await this.classRepo.findOne({ where: { id: classId } });
+    if (!cls) throw new NotFoundException('班级不存在');
+
+    const students = await this.studentRepo.find({ where: { id: In(uniqueStudentIds) } });
+    if (students.length !== uniqueStudentIds.length) {
+      throw new NotFoundException('部分学生不存在');
+    }
+
     const existing = await this.classStudentRepo.find({
-      where: { classId, studentId: In(studentIds) },
+      where: { classId, studentId: In(uniqueStudentIds) },
     });
-    const existingIds = new Set(existing.map((e) => e.studentId));
-    const newIds = studentIds.filter((id) => !existingIds.has(id));
-
-    const entries = newIds.map((sid) =>
-      this.classStudentRepo.create({
-        classId,
-        studentId: sid,
-        joinDate: new Date().toISOString().split('T')[0],
-      }),
+    const existingByStudentId = new Map(
+      existing.map((classStudent) => [classStudent.studentId, classStudent]),
     );
-    if (entries.length) await this.classStudentRepo.save(entries);
+    const today = new Date().toISOString().split('T')[0];
+    let skipped = 0;
+    const memberships = uniqueStudentIds.flatMap((studentId) => {
+      const current = existingByStudentId.get(studentId);
+      if (current?.status === 'active') {
+        skipped++;
+        return [];
+      }
+      if (current) {
+        current.status = 'active';
+        current.joinDate = today;
+        current.leaveDate = null;
+        return [current];
+      }
+      return [
+        this.classStudentRepo.create({
+          classId,
+          studentId,
+          status: 'active',
+          joinDate: today,
+        }),
+      ];
+    });
+    if (memberships.length) await this.classStudentRepo.save(memberships);
 
-    return { added: entries.length, skipped: studentIds.length - entries.length };
+    return { added: memberships.length, skipped };
   }
 
   async removeStudent(classId: number, studentId: number) {
-    await this.classStudentRepo.delete({ classId, studentId });
+    const membership = await this.classStudentRepo.findOne({
+      where: { classId, studentId },
+    });
+    if (!membership) throw new NotFoundException('学生不在该班级');
+    if (membership.status !== 'active') throw new BadRequestException('学生已离班');
+
+    membership.status = 'left';
+    membership.leaveDate = new Date().toISOString().split('T')[0];
+    await this.classStudentRepo.save(membership);
     return { success: true };
   }
 
diff --git a/apps/server/src/database/database-migrations.class-student.spec.ts b/apps/server/src/database/database-migrations.class-student.spec.ts
new file mode 100644
index 0000000..6088762
--- /dev/null
+++ b/apps/server/src/database/database-migrations.class-student.spec.ts
@@ -0,0 +1,56 @@
+import { Test, TestingModule } from '@nestjs/testing';
+import { getDataSourceToken } from '@nestjs/typeorm';
+import { DatabaseMigrationsService } from './database-migrations.service';
+
+function createRunner(tableExists: boolean, columns: string[] = []) {
+  return {
+    connect: jest.fn(),
+    release: jest.fn(),
+    getTables: jest.fn().mockResolvedValue(tableExists ? [{ name: 'class_student' }] : []),
+    getTable: jest.fn().mockResolvedValue({
+      name: 'class_student',
+      columns: columns.map((name) => ({ name })),
+    }),
+    dropColumn: jest.fn().mockResolvedValue(undefined),
+  };
+}
+
+async function createService(runner: ReturnType<typeof createRunner>) {
+  const module: TestingModule = await Test.createTestingModule({
+    providers: [
+      DatabaseMigrationsService,
+      {
+        provide: getDataSourceToken(),
+        useValue: {
+          options: { type: 'better-sqlite3' },
+          createQueryRunner: jest.fn().mockReturnValue(runner),
+        },
+      },
+    ],
+  }).compile();
+  return module.get(DatabaseMigrationsService) as DatabaseMigrationsService & {
+    removeUnusedClassStudentColumns(): Promise<void>;
+  };
+}
+
+describe('DatabaseMigrationsService — class student cleanup', () => {
+  it('drops the unused enrollment_id column', async () => {
+    const runner = createRunner(true, ['id', 'enrollment_id']);
+    const service = await createService(runner);
+
+    await service.removeUnusedClassStudentColumns();
+
+    expect(runner.dropColumn).toHaveBeenCalledWith('class_student', 'enrollment_id');
+    expect(runner.release).toHaveBeenCalled();
+  });
+
+  it('does nothing when the table is absent', async () => {
+    const runner = createRunner(false);
+    const service = await createService(runner);
+
+    await service.removeUnusedClassStudentColumns();
+
+    expect(runner.dropColumn).not.toHaveBeenCalled();
+    expect(runner.release).toHaveBeenCalled();
+  });
+});
diff --git a/apps/server/src/database/database-migrations.service.ts b/apps/server/src/database/database-migrations.service.ts
index 0254339..d9c8587 100644
--- a/apps/server/src/database/database-migrations.service.ts
+++ b/apps/server/src/database/database-migrations.service.ts
@@ -16,6 +16,7 @@ export class DatabaseMigrationsService implements OnApplicationBootstrap {
     await this.protectAttendanceHistory();
     await this.removeUnusedClassroomColumns();
     await this.cleanupDepositRefundColumns();
+    await this.removeUnusedClassStudentColumns();
   }
 
   private async removeUnusedClassroomColumns(): Promise<void> {
@@ -75,6 +76,22 @@ export class DatabaseMigrationsService implements OnApplicationBootstrap {
     }
   }
 
+  private async removeUnusedClassStudentColumns(): Promise<void> {
+    const runner = this.dataSource.createQueryRunner();
+    await runner.connect();
+    try {
+      const tables = await runner.getTables(['class_student']);
+      if (tables.length === 0) return;
+
+      const table = await runner.getTable('class_student');
+      if (table?.columns.some((column) => column.name === 'enrollment_id')) {
+        await runner.dropColumn('class_student', 'enrollment_id');
+      }
+    } finally {
+      await runner.release();
+    }
+  }
+
   private async ensureAiConfigTable(): Promise<void> {
     const runner = this.dataSource.createQueryRunner();
     await runner.connect();
diff --git a/apps/server/src/entities/class-student.entity.ts b/apps/server/src/entities/class-student.entity.ts
index 4b9906c..7a6ca73 100644
--- a/apps/server/src/entities/class-student.entity.ts
+++ b/apps/server/src/entities/class-student.entity.ts
@@ -30,14 +30,11 @@ export class ClassStudent {
   @JoinColumn({ name: 'student_id' })
   student: Student;
 
-  @Column({ name: 'enrollment_id', type: 'integer', nullable: true })
-  enrollmentId: number;
-
   @Column({ name: 'join_date', type: 'date', nullable: true })
-  joinDate: string;
+  joinDate: string | null;
 
   @Column({ name: 'leave_date', type: 'date', nullable: true })
-  leaveDate: string;
+  leaveDate: string | null;
 
   @Column({ name: 'status', length: 10, default: 'active' })
   status: string;