public void Register(Timer t)
 {
     t.Time+= Ringing;
 }
 public void Unregister(Timer t)
 {
     t.Time -= StudentMessage;
 }
 public void Unregister(Timer t)
 {
     t.Time -= Ringing;
 }
 public void Register(Timer t)
 {
     t.Time += StudentMessage;
 }