actually if you look at a lot of models and stuff they have really long hair too, past the shoulder blades, like their longest layer is mid-back. you could do big curls like a sexy victoria's secret model, or do a super super straight look (i like this look for formals, it more contemporary and edgy especially with a cute cocktail-ish kind of dress.) Also, a lot depends on the kind of dress you're wearing ...
if you're wearing a princess/ball gown kind of prom dress go curls. if its a strapless/cocktail/more sleek look the straigh hair would be cute, maybe do the puff thing (i don't know the name of it) with a pretty clip or something.
http://media.kansascw.com/images/ANT...%5B1%5D.a1.jpg