private void cmbGrade_SelectedIndexChanged(object sender, EventArgs e)
        {
            ClassController classController = new ClassController();

            if (cmbGrade.SelectedValue == null)
            {
                ;
            }
            else
            {
                var listClass = classController.GetListClass((decimal)cmbGrade.SelectedValue);
                if (listClass.Count > 0)
                {
                    cmbClass.DataSource = listClass;
                }
            }
        }