private bool IsLessonTeacher(DateTime date, LessonsOrder lesson, Teacher teacher)
 {
     bool b = false;
     if (items.ContainsKey(date))
     {
         foreach (ScheduleItem s in items[date])
         {
             if (s.Lesson == lesson && s.Teachsrs.Contains<Teacher>(teacher))
             {
                 b = true;
                 break;
             }
         }
     }
     return b;
 }
 public void AddTeacher(Teacher t)
 {
     teachers.Add(t);
 }