Not going to senior prom, what am I missing?

Simply put, if you had a date, or a lot of friends who were going to prom, it would definitely be worth going. If you don't know that many people at the school, you don't have a strong connection to the people you do know, and/or you don't have a date, you're missing nothing. You could go. You'd spend a few hours talking with people you don't know that well and watch the occasional drunk couple falling down or each other in a corner. If that really interests you, then go. Otherwise, don't bother, since you have no real interest in/reason for going.