private void btnAddStudent_Click(object sender, EventArgs e)
        {
            using (var addItem = new AddItem("student"))
            {
                addItem.ShowDialog();
            }

            getData();
            updateGUI();
        }
        private void btnAddTeacher_Click(object sender, EventArgs e)
        {
            using (var addItem = new AddItem("teacher"))
            {
                addItem.ShowDialog();
            }

            getData();
            updateGUI();
        }