To everyone who picked schoolgirl: That is SO overated. x_X Most girls dress like that on a daily basis anyhow.
I say...be original! Put something together with things you find interesting. I personally took various parts of different costumes which I found appealing. I am going as some sort of gothic vampire chick with a punk outfit...pink/black leg stockings and all, haha! I am most curious about wearing the new vampire teeth. XD
You're waiting kind of close to Halloween to get a lot of the better things, but go look around and see what you can come up with. ^_^ If all else fails...
I say don't buy into the whole "dress like a prostitute for Halloween" thing. Like Mecha said, creativity is better anyway. You'll catch more eyes showing a lot of skin, but are they really the eyes you'd want to catch?