The best cheating I ever saw in highschool/university was:

1. written on the brim of a hat sitting on the table. Guy would “fiddle” with it while “thinking hard”.

2. guy peeled off a water bottle label, wrote on it, and stuck it back. You could read the notes through the bottle, but they were faint enough it wasn’t too obvious waking by.

I sometimes had a very small piece of paper scribbled with stuff I couldn’t remember while learning but turns out this way I knew what I had written down there and didn’t need it afterall :D

One time I accidentally forgot about that tiny paper though and my teacher actually found it on some stairs and recognized my handwriting.