sub query count laravel

 public function classes($teacherId)
    {
        $teacherClass =
            TeacherClass::join('m_class','m_class.id','m_teacher_class.class_id')
                ->select('m_teacher_class.class_id','m_class.name as name_class',
                    DB::raw("(select COUNT(*) from m_student where m_student.class_id = m_teacher_class.class_id ) as student")
                )
                ->where('teacher_id',$teacherId)->get();
        dd($teacherClass);
    }