If you're buying off the rack, you're really likely to have the same dress as at least one other person. I ordered a senior ball dress off the internet, and I saw at least two other people with my dress on. I went to a small school, too. If you go with something vintage, you have less of a chance that someone will be your dress twin.
Long dresses out of a shiny material are generally what people wear to prom. Sometimes they're figure-hugging, other times the dresses have full skirts. Most prom dresses don't have sleeves.
Even though prom dresses have become "sexier", I would advise against it. Your parents will probably want to see what you're wearing, and your teachers are probably going to be chaperones; I think it would be odd to have half your chest hanging out of a dress while your English teacher circles the room. Also, a lot of schools have strict rules about what you can and can't wear. Then again, one of the dances had rules about nudity and someone did the Lil' Kim pasty thing.