private void DGV_RowHeaderMouseClick(object sender, DataGridViewCellMouseEventArgs e)
        {
            try
            {
                DataGridViewRow dr = DGV.SelectedRows[0];
                if (dr.IsNewRow || string.IsNullOrEmpty(dr.Cells[0].Value.ToString()))
                {
                    return;
                }

                frmTeacherSubjectEntry frm = new frmTeacherSubjectEntry();
                frm.adding = false;
                frm.Show();
                frm.cmbClass.Text   = dr.Cells["classname"].Value.ToString();
                frm.cmbSubject.Text = dr.Cells["subjectname"].Value.ToString();
                frm.cmbStaff.Text   = dr.Cells["staffname"].Value.ToString();
                frm.btnDelete.Tag   = dr.Cells["id"].Value.ToString();
                GroupBox1.Tag       = "Teacher: " + dr.Cells["staffname"].Value.ToString() + "  Subject : " + dr.Cells["subjectname"].Value.ToString() + " ";
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void btnAddNew_Click(object sender, EventArgs e)
        {
            frmTeacherSubjectEntry frm = new frmTeacherSubjectEntry();

            frm.Show();
        }