private void RemoveClass(ViewItemClass c)
 {
     if (TrackChanges)
     {
         _addedClasses.RemoveWhere(i => i == c);
         base.StopObservingCollection(c.Schedules);
         c.PropertyChanged -= _classPropertyChangedEventHandler;
     }
     IListExtensions.RemoveWhere(this, i => i.Class == c);
 }