private void btnAddStudent_Click(object sender, EventArgs e)
 {
     Form1.Student AuxStudent = new Form1.Student();
     AuxStudent.DNI  = txtDNIstudent.Text;
     AuxStudent.name = txtNameStudent.Text;
     Principal.Estudiantes.Add(AuxStudent);
 }
 private void btnValidateStudent_Click(object sender, EventArgs e)
 {
     Form1.Student AuxStudent = new Form1.Student();
     AuxStudent = Principal.Estudiantes.Where(p => p.DNI == txtDNIStudent.Text).FirstOrDefault();
     if (AuxStudent != null)
     {
         txtDisplayDNI.Text    = AuxStudent.DNI;
         txtDisplayNombre.Text = AuxStudent.name;
         EstudianteActual      = AuxStudent;
     }
     else
     {
         MessageBox.Show("el estudiante no exite");
         txtDNIStudent.Focus();
     }
 }