Since you are a senior, you and most of your friends should be driving by now, get a hold of everyone that drives and park in the faculty parking lot really early in the morning so the staff and faculty can't part anywhere.
does your school have more than one floor? if so put a cow up on the second floor. cattle can not go down stairs so it will be hard as hell to get it down..... (I doubt it will get in an elevator either)
just a thought on getting into the school. super glue the latching system on all the doors. that way they get locked, but never latched. i suggest you test it first to see if it works before you bet your hole plan on it and it fails.
it's a 2 part glue, comes in the double syringes, you mix the two parts, one is pretty thick, the other is yellow and smells like puppy piss, and then they mix and they harden like, fuckin hard and stick to anything.
personally me and some friends are going to epoxy all of the locks, and pour some fucking cement blocks randomly in all of the parking lots other than the senior parking lots (teacher and lower classmen)
for the cup with water thing to work you have to staple all of the cups together. Just prepare them in large blocks ahead of time, and staple the pre-made blocks together when you get there.
if you go to a public school all you have to do to get school canceled is tear out pages of the bible and put them around, and use sharpie to write verses. the public systems cannot have religon present in a school enviroment.