FATBABY hit it right on the head; essentially, you're not missing much. Overpriced food and a date that thinks you're made of money.
I went to a HS that was a 20-minute drive from the beach. In my school, the Prom Tradition was to do the prom, go out on the town and somehow wind up on the beach. You would spend the night on the beach, getting drunk, having sex with your date.
Then, the next morning, after singing songs and doing acts of superhuman sexual strenght, everyone meets up at the local diner for breakfast. Whatever.
My prom night (I DID get laid, though!!!! Mike always gets a piece, dammit!

)
1. Go to prom (already spent like $200 on the limo, $100 on the tux and $20 on the corsage)
2. Go out on town (another $200 real quick)
3. Go to beach....uh no...it was hella cold out there!!!!! Ditch rest of gang...go to the NoTell Motel w/date. Do the nasty. Three times (I was 17....i miss my libido)
4. Take her directly home after that b/c neither one of us could keep our eyes open, see straight or remember what the hell just happened. Also, neither one of us had our "own place" You don't become cool until after you're an adult and can afford your own damn place!!

5. In bed in Mommy's house and asleep by about 0700 hours.
And your Prom?