I know a school in my area is a legond for the pranks they have pulled off
maybe the Cypress Creek High School members of team 665 might be able to help you out
a.l.a.
http://news.bbc.co.uk/2/hi/americas/8014324.stm (this years prank recived international attention)
last year they let chickens loose across campus, and the year before that, they decided to detach the outside coutryard lunch area furniture and turn it into a parymid.....
there are MANY pranks i can think of that would work, but not knowing how strict your administration is leaves a huge ? (tip a cow in a hallway/fish in vent system/rubber cement on light switches that requre a key.... doing it while their off,)