Reflection method

The day before your test take a folder with a relatively reflective surface. Scribble the necessary notes on the surface using a sharp piece of metal (a pair of compasses for example). Make sure it is in small writing. The next day bring that folder on your test (if asked say it is revision for some other subject, you may actually consider putting some inside of it). Put the folder on the top of the other ones and look at it from the right angle. The notes will show up. You can also use a ruler like that instead of a folder. Don't tell too many people about it, or it may become suspicious.