This one mostly works when you're sitting alone and away from the teacher.

If you have a cheat sheet for a test and your teacher lets you have your phone on you, set the cheat sheet as your home wallpaper but not your lock wallpaper. Leave the phone on your desk screen-up so you can look at the answers, but don't touch or scroll the screen. That way, if your teacher is coming, you can just shut off your phone and pretend you weren't doing anything.