Wow, you guys have no ****ing imagination.
There's so much awesome fun stuff you could do if you could freeze time. Just because you can't do any really illegal stuff doesn't mean you can't do really elaborate, but utterly harmless pranks.
I'm talking about causing mass confusion here. What about freezing time in school and turning every desk and large object you can possibly find upside down. Or lifting everyones pencil/pen/whatever several inches in the air and unfreezing time to have them drop simultaneously all over the building. How about planting some cd players and mini speakers in hidden places, maybe inside the ceiling, and have them blast some really annoying music (or perhaps use this opportunity for mass rickrolling).
If you're really patient, how about something far more elaborate. Pick a random building with a nice flat wall on one side, and paint an enormous mural depicting Captain Crunch on a unicycle.
Ok, so maybe some of those pranks are bordering on illegal, but at least I'm being more creative than "RAEP PPL BECASS I WANT DO SEX W/ CHIX"
There's still plenty of cool stuff you could do that doesn't involve any pranking whatsoever.
How about stuff that would just look cool. Throw a large rock or something into water and freeze time in mid splash. Or set off fireworks or something and attempt to freeze time at the precise moment they explode. Or set up an epic coke and mentos fountain. Arrange hundreds of bottles of coke however you see fit, freeze time, and place a mentos hovering above each and every bottle.