Talk to your date. Find out if she has any style/colour/cut preferences? Does the school have any clothing rules?
If you're not going to wear a tux or suit, you're kind of stuck with a dress - unless you go with a jumpsuit. You still have to hit that formal/semi-formal note that skirt/tops just don't have.
Have you considered a modified one colour or white tux style outfit?
here are some wedding suits that might be fun to modify to your own taste
there's a jumpsuit on this page
google prom jumpsuit - there's some great stuff out there
I would have loved a jumpsuit option when I was of prom age.