Enter the stall, fully clothed...

 Enter the stall, fully clothed. Do not undress and make sure your clothes get all wet and soapy. Complain when leaving the bathroom that your shirt tends to bleed.
http://www.101funjokes.com/dorm-jokes.htm

Bring in a fake finger....

Bring in a fake finger. Float it down the drainage "ditch." Ask if someone would be so kind as to return it to you. If no one does, tell them that the finger has been sacrificed to Satan and that the shower stalls are now possessed. Hang Halloween decorations and crepe-paper ghosts from them the next day.

http://www.jokes-db.com/jokes/dorm-jokes

Initiate a war with the person ...

Initiate a war with the person in the stall next to you. Use the residual water on the floor as your battle medium, and float little battleships over to their side. If they kick them back or throw them over the edge, exclaim that you didn`t know they had the power of God and sheepishly mumble prayers for the duration of your shower.

http://www.jokes-db.com/jokes/dorm-jokes

Stand in the bathroom...

Stand in the bathroom, waiting for would-be shower-goers. When they come in, tell them "not to do it" and ask them "not to give in to sin." Wail mournfully when they step into the shower.

http://www.jokes-db.com/jokes/dorm-jokes

Have a seizure...

Have a seizure. Bang against the walls of the stall really hard. Try to knock them down. If anyone later asks if you are okay, just say that you had some Mexican Jumping Fava Beans and they were reacting negatively with your stomach.