What would I (bi girl) wear to prom? (going with a girl in a dress)

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

http://www.gurl.com/2015/04/06/awesome-prom-outfits-for-girls-who-hate-dresses/#1

google prom jumpsuit - there's some great stuff out there








I would have loved a jumpsuit option when I was of prom age.

Google same-sex weddings (F/F of course) and see how they do it. Of course it's not wedding gowns, but it should give you some ideas.

In the interests of full disclosure, I work for this blog, but here's a same-sex wedding we covered, and the brides were both just so lovely.
http://reveriegallery.com/colorful-same-sex-wedding/