private void cmbSchCode_SelectedIndexChanged(object sender, EventArgs e)
        {
 
            String connectionString = System.Configuration.ConfigurationSettings.AppSettings["dsn"];
 
 
            DataTable dt = new DataTable();
            string Query = String.Format(@"SELECT RegNo FROM Student WHERE SchoolCode ={0} ", cmbSchCode.Text);
            OleDbDataAdapter adapter = new OleDbDataAdapter(Query,connectionString);
 
            adapter.Fill(dt);
 
 
 
           foreach (DataRow dr in dt.Rows)
            {
                cmbStudent.Items.Add(dr[0].ToString());
            }
 
 
        }