Prom is a traditional event, and guys and girls wear traditional clothes. Girls were dresses. Period. If a girl is uncomfortable in a dress, she should still give it a try. People will give her loads of compliments about how great she looks, especially if she doesn't usually wear dresses.
True, you should wear what you want to Prom. However, in my experience the only people that wear slacks to Prom that are girls are either Teachers or Lesbians. The lesbians also compliment the slacks with a suit, bow tie, and vest. When a girl wears slacks to Prom it's not "bad" it's just not normal. Girls wear dresses and Guys wear tuxedos, it's really about matching if anything! Think about it this way, what could a girl wear with slacks to Prom? Slacks are more for a workplace setting when one wants to look professional and in control. Dresses have always been meant for dances because of the freeness and carelessness that comes with dancing.
Times have changed believe it or not, and surprisingly enough women wear trousers too. If you want to wear a suit to prom you should, no matter your gender, and the same goes for dresses, if you want to wear a dress then wear one (yes, even if you are a guy). Screw gender norms.
It’s your free choice to wear anything, anywhere. Slacks can totally be appropriate for prom. The worst reason to do something is because everyone else is doing it-so why use that reason to go in something you feel uncomfortable and negatively self-aware in? I say wear what you want; after all, it’s your prom!