private void btnAssign_Click(object sender, EventArgs e)
{
var studentId = Guid.Parse(((KeyValuePair <string, string>)lstStudent.SelectedItem).Key);
var classId = Guid.Parse(((KeyValuePair <string, string>)lstClass.SelectedItem).Key);
var res = StudentService.AssignStudent(studentId, classId);
MessageBox.Show(res.Message);
LoadData();
}