This prank should only be used against people you REALLY REALLY hate....
Now go to the local grocery store or something and buy a bottle of FAST-ACTING laxatives. Next day at school go to the cafeteria where your victim eats his/her lunch. Get close enough to their drink and pour ALOT of laxatives in it. Next thing you know there running off to the washroom all buck wild hoping to god they can make it there before they shit there pants. But to add insult to injury, put POISON ivy all over the rims of the toilet seats that your victim will be sitting on.To make matters even worse, you can even smear poison ivy all over the toilet paper also!