기존 학생 계정 학년 지정/변경 기능 추가

- 학생 계정 관리 화면에 학번으로 학년을 지정/변경하는 카드+다이얼로그 추가
- 계정 생성 성공 판정을 HTTP 상태코드가 아닌 응답 status 필드로 정확히 확인하도록 수정

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/function/teacher_student_management_controller.dart

@@ -31,8 +31,8 @@ class TeacherStudentManagementController extends ChangeNotifier {
       );
       final result = jsonDecode(utf8.decode(response.bodyBytes));
 
-      if (response.statusCode == 200 || response.statusCode == 201) {
+      if (response.statusCode == 200 && result['status'] == 'success') {
-        return (true, '✅ 계정 생성 완료: ${result['message'] ?? '성공'}');
+        return (true, '✅ ${result['message'] ?? '계정이 생성되었습니다.'}');
       } else {
         return (false, '❌ 생성 실패: ${result['message'] ?? '오류 발생'}');
       }
@@ -44,6 +44,35 @@ class TeacherStudentManagementController extends ChangeNotifier {
     }
   }
 
+  /// 🏫 이미 만들어진 학생 계정의 학년을 지정/변경한다. grade가 null이면 "미배정"으로 되돌린다.
+  Future<(bool success, String message)> updateStudentGrade({
+    required String studentId,
+    int? grade,
+  }) async {
+    _isWorking = true;
+    notifyListeners();
+    try {
+      final url = Uri.parse('$baseUrl/api/users/update-grade');
+      final response = await http.post(
+        url,
+        headers: {"Content-Type": "application/json"},
+        body: jsonEncode({"studentId": studentId, "grade": grade}),
+      );
+      final result = jsonDecode(utf8.decode(response.bodyBytes));
+
+      if (response.statusCode == 200 && result['status'] == 'success') {
+        return (true, '✅ ${result['message'] ?? '학년이 지정되었습니다.'}');
+      } else {
+        return (false, '❌ 지정 실패: ${result['message'] ?? '오류 발생'}');
+      }
+    } catch (e) {
+      return (false, '🚨 네트워크 에러: $e');
+    } finally {
+      _isWorking = false;
+      notifyListeners();
+    }
+  }
+
   /// ❌ 학생 계정 삭제.
   Future<(bool success, String message)> deleteStudentAccount(
     String studentId,

lib/ui/teacher_student_management_page.dart

@@ -63,6 +63,99 @@ class _TeacherStudentManagementPageState
     ).showSnackBar(SnackBar(content: Text(message)));
   }
 
+  // 🏫 [기존 학생 학년 지정/변경 다이얼로그]
+  void _showUpdateGradeDialog() {
+    final TextEditingController idController = TextEditingController();
+    int? grade;
+
+    showDialog(
+      context: context,
+      builder: (context) {
+        return StatefulBuilder(
+          builder: (context, setDialogState) {
+            return AlertDialog(
+              shape: RoundedRectangleBorder(
+                borderRadius: BorderRadius.circular(24),
+              ),
+              title: const Text(
+                '🏫 학년 지정/변경',
+                style: TextStyle(fontWeight: FontWeight.bold, fontSize: 18),
+              ),
+              content: Column(
+                mainAxisSize: MainAxisSize.min,
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  const Text(
+                    '이미 만들어진 학생 계정의 학년을 지정하거나 바꿉니다.',
+                    style: TextStyle(color: Colors.black54, fontSize: 13),
+                  ),
+                  const SizedBox(height: 16),
+                  TextField(
+                    controller: idController,
+                    keyboardType: TextInputType.number,
+                    decoration: const InputDecoration(
+                      labelText: '학번',
+                      border: OutlineInputBorder(),
+                      prefixIcon: Icon(Icons.badge),
+                    ),
+                  ),
+                  const SizedBox(height: 12),
+                  DropdownButtonFormField<int?>(
+                    initialValue: grade,
+                    decoration: const InputDecoration(
+                      labelText: '학년',
+                      border: OutlineInputBorder(),
+                      prefixIcon: Icon(Icons.class_),
+                    ),
+                    items: const [
+                      DropdownMenuItem(value: null, child: Text('미배정')),
+                      DropdownMenuItem(value: 1, child: Text('1학년')),
+                      DropdownMenuItem(value: 2, child: Text('2학년')),
+                      DropdownMenuItem(value: 3, child: Text('3학년')),
+                    ],
+                    onChanged: (value) => setDialogState(() => grade = value),
+                  ),
+                ],
+              ),
+              actions: [
+                TextButton(
+                  onPressed: () => Navigator.pop(context),
+                  child: const Text('취소', style: TextStyle(color: Colors.grey)),
+                ),
+                ElevatedButton(
+                  onPressed: () async {
+                    final inputId = idController.text.trim();
+                    if (inputId.isEmpty) return;
+                    Navigator.pop(context);
+                    final (_, message) = await _controller.updateStudentGrade(
+                      studentId: inputId,
+                      grade: grade,
+                    );
+                    if (!mounted) return;
+                    ScaffoldMessenger.of(
+                      context,
+                    ).showSnackBar(SnackBar(content: Text(message)));
+                  },
+                  style: ElevatedButton.styleFrom(
+                    backgroundColor: Colors.blue,
+                    foregroundColor: Colors.white,
+                    shape: RoundedRectangleBorder(
+                      borderRadius: BorderRadius.circular(12),
+                    ),
+                  ),
+                  child: const Text(
+                    '적용',
+                    style: TextStyle(fontWeight: FontWeight.bold),
+                  ),
+                ),
+              ],
+            );
+          },
+        );
+      },
+    );
+  }
+
   // ❌ [학생 삭제 버튼 동작]
   Future<void> _deleteStudentAccount(String studentId) async {
     final (_, message) = await _controller.deleteStudentAccount(studentId);
@@ -284,6 +377,77 @@ class _TeacherStudentManagementPageState
                 ),
                 const SizedBox(height: 36),
 
+                // 🏷️ 인디케이터 바 (기존 학생 학년 지정/변경)
+                Row(
+                  children: [
+                    Container(
+                      width: 4,
+                      height: 16,
+                      decoration: BoxDecoration(
+                        color: Colors.blue,
+                        borderRadius: BorderRadius.circular(2),
+                      ),
+                    ),
+                    const SizedBox(width: 8),
+                    const Text(
+                      '기존 학생 학년 지정/변경',
+                      style: TextStyle(
+                        fontSize: 18,
+                        fontWeight: FontWeight.bold,
+                      ),
+                    ),
+                  ],
+                ),
+                const SizedBox(height: 16),
+
+                InkWell(
+                  onTap: _showUpdateGradeDialog,
+                  borderRadius: BorderRadius.circular(24),
+                  child: Container(
+                    padding: const EdgeInsets.all(20),
+                    decoration: BoxDecoration(
+                      color: Colors.blue[50],
+                      borderRadius: BorderRadius.circular(24),
+                      border: Border.all(color: Colors.blue.shade200),
+                    ),
+                    child: const Row(
+                      children: [
+                        Icon(Icons.class_rounded, color: Colors.blue, size: 32),
+                        SizedBox(width: 16),
+                        Expanded(
+                          child: Column(
+                            crossAxisAlignment: CrossAxisAlignment.start,
+                            children: [
+                              Text(
+                                '학번으로 학년 지정',
+                                style: TextStyle(
+                                  fontSize: 16,
+                                  fontWeight: FontWeight.bold,
+                                  color: Colors.blue,
+                                ),
+                              ),
+                              SizedBox(height: 2),
+                              Text(
+                                '이미 만든 계정의 학년을 나중에 지정하거나 바꿀 때 사용',
+                                style: TextStyle(
+                                  fontSize: 12,
+                                  color: Colors.blueAccent,
+                                ),
+                              ),
+                            ],
+                          ),
+                        ),
+                        Icon(
+                          Icons.arrow_forward_ios_rounded,
+                          color: Colors.blue,
+                          size: 16,
+                        ),
+                      ],
+                    ),
+                  ),
+                ),
+                const SizedBox(height: 36),
+
                 // 🏷️ 인디케이터 바 2 (삭제 권한 제어메뉴)
                 Row(
                   children: [