It would be kind of funny, if you could hook up some speakers so that when she/he opened the locker they would start playing some embarrassing song like miley cirus etc, and would be even better, if you got a cheap strobe light to flash also so people knows whose it is.
Or, you should get an old text book, and get some Salmon, and slice it really thin and start placing slices of it in between pages and super gluing them together so you cant see it. I think it would need to be in between pages with pictures since if it was just white paper with words u would see it. But do that and give it a week, and his locker will smell so bad and he will have no clue why.