private void cbxClassName_SelectedIndexChanged(object sender, EventArgs e)
{
string cbx = cbxClassName.SelectedValue.ToString();
DataTable dt = StudentDAO.GetListSubjectByClass_Quang(cbx);
lbxSubject.DataSource = dt;
lbxSubject.ValueMember = "SubjectID";
lbxSubject.DisplayMember = "SubjectName";
}