Follow the rules
You may think that you can only use rulers in math class. But think about it – they offer a long surface on which you can write all kinds of stuff. Especially formulas, but you can also write answers to other exams on them.
The best part is that you can totally re-use them. That doesn’t count as cheating; it is taking care of the environment; instead of ruining countless pages to write your cheat sheets on, you are always writing on the same ruler. There is no reason to think that it is out of place on top of your desk.