Dude, those are awesome. I have this book of pranks at home, but none of them are really on a grandiose school prank level. Unless.......

If your school has showers, sneak in when no one's around, take off all the shower heads, and put bouillon cubes in all of them. Then put them back, so it looks as if nothing is amiss. When sweaty athletes come in for a shower-off, they will instead be greeted by beef/chicken/pork sludge.