private void btQuanLySinhVien_Click(object sender, EventArgs e)
        {
            DanhSachSinhVien DSSV = new DanhSachSinhVien();

            DSSV.TopLevel = false;
            HienThi.Controls.Clear();
            HienThi.Controls.Add(DSSV);
            DSSV.Show();
        }
 private void btQuanLySinhVien_Click(object sender, EventArgs e)
 {
     DanhSachSinhVien DSSV = new DanhSachSinhVien();
     DSSV.TopLevel = false;
     HienThi.Controls.Clear();
     HienThi.Controls.Add(DSSV);
     DSSV.Show();
 }