private void btnEditStudent_Click(object sender, EventArgs e)
 {
     StudentIndex = listBoxStudents.SelectedIndex;
     FormStudent fs = new FormStudent(this);
     fs.ShowDialog();
 }
 private void btnAddStudent_Click(object sender, EventArgs e)
 {
     studentIndex = -1;
     FormStudent fs = new FormStudent(this);
     fs.ShowDialog();
 }