Although there wasn't a teacher in the cupboard, we would convince the child that there was before locking him in there.
That was the cue for the other pupil who was in the cupboard to start hurling ink and paper at him, and shout teacherly things like "You, BOY!" at him.
Surprising fun for those outside the cupboard, who got to hear the shouting and watch a cupboard rattle around for a bit.

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.

We always used to hide in a small storage room in an adjoining attic room before the teacher arrived. Once, after our usual cheerful parade into the classroom, having got yet another good-natured one over on Sir, he noticed that one boy hadn't come out, and tried to coax him out with increasing frustration for over five minutes.

What we knew, that the teacher didn't, was that the child was off sick.