Class dismissed

Can anyone tell me why, in a predominance of scenes in movies/TV where a class lecture is being given, the lecturer always seems to be starting his/her lecture, only to have the class end 2 minutes later? What the heck was the teacher doing the first 58 minutes?

It's like, "Today's lecture is on George Washington, our first President. George Washington was a great.. oops, class dismissed. Please read chapters 2 and 3 for next time. Oh, and sorry about the beginning of class there, today's Jerry Springer was too good to miss!"