Many of my tests after a certain point were open book, or at least explicitly allowed a cheat sheet.

It wasn't going to help you if you didn't know the material, though writing the cheat sheet in the first place is a pretty good form of studying.

We made grids of tiny dots on an eraser, so tiny a grownup couldn't really see it.

End of the year a teacher asked how we did it. He knew our answers were the same, he put us far apart, he watched us the whole time every test, the whole year.

We asked for an eraser then someone "random" would throw it then asked for it te be returned when they needed it again.

I pointed at a tiny dot on the eraser. Roughly 3x2 mm. There, that was the last test. He couldn't see it.