private void ThemHocSinhVaoLop_Load(object sender, EventArgs e)
        {
            List <string> lops = Lbus.GetDanhSachLopString();

            lops.Insert(0, "None");
            comboBoxLop.DataSource         = lops;
            comboBoxGioiTinh.SelectedIndex = 0;
            hocSinhs = HSbus.XemTatCaHS();
            dataGridViewDS_HS.DataSource = hocSinhs;
            AddCotCheckbox();
            CheckHocSinh();
        }