Ah, the trick here is to do several small things to them.
Put bleach in their shampoo.
Put cooking oil in the sunscreen (or tanning oil).
Put pepper spray in their makeup remover.
Put some salt in the sugar.
put some vinegar in the water.
And one of my favorites, leave a bagel under one of their beds so they get ants (you don't want you girlfriend here for that, as the whole dorm room may become infested).
Mix the cinnamon toast crunch with the reeses puffs (two conflicting flavors!) so that they either have you eat their cereal mixed, or pain-stakingly sift through and seperate them.
Stick your dick in the jelly jar.
While they're sleeping, stick band-aids on random places on their bodies (believe me, it really gets in their heads).
(Also when they're sleeping) Open they're windows, and then claim that you didn't open their windows.
(If you have access to a dead mouse) Leave a dead mouse somewhere in their room, but make it somewhere obvious, so they notice it right away.
Then, go back and stick your dick in the jelly jar AGAIN!