In the dorm where I live (Fowler Hall, for you Aggies out there in noderland), each bathroom is shared between two adjoining rooms. Since it would be simple to walk through the bathroom into your suitemate's room, the bathrooms are fitted with locks on the outside of the door. A simple joke is to wait until someone goes into the bathroom, then lock him in. Immature, simplistic, and surprisingly funny.

A more advanced trick: tie a rope to the inside of the opposite door in the bathroom, then secure the other end of the rope to an immobile object in your room (beds work well for this). Now wait until one of guys in the other room to hear "nature's call" and try to open the bathroom door.

Breaking and entering: Enlist your victim's suitemates in this one. The so-called closets in each room sit back-to-back, and they have one positively huge drawer in the bottom of each closet. Remove one of the drawers in your co-consprator's room, and slither into the empty space. Push out the victim's drawer, use a long ruler to frob the drawer retaining clips if you need to. Pust the victim's drawer out onto the floor, and push it aside. You should now be able to crawl through the gap into his room.

Highly immature: put kool-aid into the air vents, that room will smell like kool-aid forever.

Sleep deprivation torture: herd, chase, push, or otherwise coerce live crickets under the door and into the victim's room.

Devious, but quickly discovered: wait for your roomate to go to class. Crack rommate's windoze screensaver password. Replace his wallpaper with gay pr0n. When he returns it will be a kodak moment. (just ask Ender02 about this one :-)