public function index(Request $request)
 {
     $classrooms = Classroom::orderBy('name', 'desc');
     if ($request->has('teacher_id')) {
         $classrooms->where('teacher_id', $request->input('teacher_id'));
     }
     if ($request->has('subject_id')) {
         $classrooms->where('subject_subject_id', $request->input('subject_id'));
     }
     if ($request->has('student_id')) {
         $classrooms->whereHas('students', function ($query) {
             return $query->where('students.id', request('student_id'));
         });
     }
     $classrooms = $classrooms->inCurrentSemester()->get();
     return $classrooms;
 }