OK first your going to need one of those card when you open it it makes a really annoying sound and of course an incredible whinny victim.
Step 1: What to Do
OK so know cut out the nouse making part and glue it into the door of your victims locker or room and wait.
Step 2: Trolling Part!=)
OK so hopefully your victim has opened his door/locker and is wondering what is making the noise then come up behind him and say "hey man whats up?" and he will hopefully ask back "can't you hear it?" and you will say "no you must be hearing things".
Step 3: Time to Tell Him=(
OK it's probably getting boring after about 5 days so i would recommend telling him it was you or just remove it when he is not there and leave him forever wondering.
p.s try not to get killed when he finds out.