Also, you must have a uninterrupted, undirected preparation period every day. You cannot cover a class if doing so would eliminate your only preparation period for the day.
Middle and high school teachers must cover 4 classes during the school year without receiving compensation. Elementary teachers must cover 2 classes without receiving compensation.
Also, if you are a mentor or protege, you do not have to cover classes. But, if you do, you are compensated every time.