You shouldn't have sex with her only because of prom. I mean, if you two want to have sex, then go for it. but don't make prom the reason behind the sex.
Nope, not a tradition, besides you should have sex b/c you want to, not because it's "tradition". The mood has to be just right, if it happens, it happens, if not, then you'll get over (you have a hand don't ya?)