dude, that union thing around here kicks major butt. if you get fired for something stupid like taking the day off for prom, you'll get rehired in a jiffy.
that is really stupid if he does fire you. if he does, im sure youre still young and can find another job.
in fact, if he even says something like "well im going to have to let.." jump in and just say , you know what I QUIT, you cant fire me CUZ I QUIT!
i hope youre able to go to your prom. i never went to mine, but i know its hella fun.
enjoy it!