Here is a mean easy prank you can do in gym just dont try it because you'll get in lots of trouble. LOTS of trouble.
Step 1: Materials
The materials you need are: Icy hot
Step 2: Instructions
All you have to do is put the Icy hot in the back of their gym shorts.
Step 3: Done
Now all you have to do is watch as it starts to warm up. Eventually it will start to get hotter, then hotter, then hotter!!