This isn't always a lie. Our German teacher went to the special effort of hiding in a cupboard at the start of the lesson, to see what people said about him.
It's a well-established moral to many tales, that if you have the ability to read minds, you must be willing to hear things you don't like. Such as thirty children sagely discussing exactly how much of a cunt you are, not to mention a bit late.
Our teacher wasn't nearly as philosophical about it; he burst out of the cupboard, shouted for ten minutes, and put us all in detention.