Yes, I'm rater late. It just came up on latest posts and I can't see existing answers unless I remember to tab down to check.
I have quite a stock of maths based mind reading tricks, but one simple one that nearly always works when I do supply work is to ask the cockiest looking boy in the room to give me a number between 1 and 100. The cocky lad thinks for a second then smirks and gives me his number. Then, 9* times out of 10 I can, without writing anything, produce a large piece of paper from my pocket which bears his number written in words. What do you think the number is?
(*If you're wondering what happens on the 1 in 10 times I'm wrong, I don't reveal the sheet and simply use his number to start another trick)