private void button5_Click(object sender, EventArgs e)
        {
            this.Hide();
            Form ds = new cboLop();

            ds.Show();
        }
        private void lvChucnang_DoubleClick(object sender, EventArgs e)
        {
            string ItemName = lvChucnang.SelectedItems[0].Text;

            switch (ItemName.ToLower())
            {
            case "tiep nhan hoc sinh moi":
                Form1 frm = new Form1();
                frm.Show();
                break;

            case "tim kiem ho so hoc sinh":
                cboLop ds = new cboLop();
                ds.ShowDialog();
                break;
            }
            this.Hide();
        }