public function assignTeacher($id, $course_id)
 {
     $teacher = new Teacher();
     $user = User::find($id);
     $school_id = $user->school_id;
     $teacher_id_num = $school_id * 100000;
     $teacher->user_id = $user->id;
     $teacher->teacher_id = $teacher->increment('teacher_id') + $teacher_id_num;
     $teacher->school_id = $school_id;
     $teacher->account_status = 'assigned';
     $enrollment = $teacher->save();
     if ($enrollment) {
         return $teacher->teacher_id;
     } else {
         return array('success' => false);
     }
 }