I agree with the above: ask your school, and/or just forget it.
Cardboard and duct tape are your friends
I remember helping a roomie in university make a dalek costume in about 2 hours with just cardboard, duct tape, tinfoil, and a whack of those chocolate gold coins....jerk got attention all night
Your friendly neighborhood Value Village is a source of GREAT costume materials on the cheap! For $20 I found a black hoodie, cape, pants, a plastic sythe, and a small plastic skeleton. I went as the grim reaper; with the skeleton over my shoulder and a small sign saying "you're next Bieber!"