2 pranks that work well:

Take a large (9x12) envelope, seal it up, then cut off a corner and fill with shaving cream. Tuck corner of envelope under door of dorm room. Drop large textbook from height on envelope.

Take about .50 in pennies. Put pressure on top of closed dorm room door to open up a gap between the door and the doorjamb and wedge with small stacks of pennies, sliding them down toward latch until they're really tight. Repeat from bottom of door. By putting enough pressure on the latch, the door CANNOT be opened from inside.