private void NewMenuItem_Click(object sender, RoutedEventArgs e)
        {
            newStudent newS = new newStudent(0);

            newS.Show();
            Close();
        }
        private void StudentList_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            int        aux  = StudentList.SelectedItem.ToString().IndexOf("-", 0);
            int        Id   = int.Parse(StudentList.SelectedItem.ToString().Substring(0, aux).ToString().Trim());
            newStudent newS = new newStudent(Id);

            newS.Show();
            Close();
        }