I think you'll have to confiscate it. The available ways to cheat I can think of already even without a phone:
- First off if you take their phone and it's still within bluetooth range it might be with an average size class room, you'll need to make sure the phones are OFF
- watch can have a photo album with cheat sheets in it
- Song names can be used as cheat notes
- some 3rd party apps might be able to have some offline data show up
- if both the watch and phone are on the same network, then they can communicate and the kid can message and make calls from the watch
- though the watch has wifi it currently can't work as a browser or anything without the iPhone, but you never know when some app comes out or apple changes things that allow the watch to do more without the iPhone.