I'm sort of on a Harry Potter fix right now, so here goes...
You could be either Hermione or Ginny or any other female Harry Potter student. You just need a short, grey, pleated skirt, some mary janes, dark knee socks, button down shirt, striped tie (color depending on house), grey sweater and a wand. That's actually what I'm being for Halloween. Not the most original, but it's got a kinda sexy schoolgirl thing going on. And most of the stuff you can probably find lying around the house or wear again.
Also, if you go to www.pinupgirlclothing.com/roleplay.htm, they sell costumes, but they've got a lot of them and you can browse through for some ideas, then try to make your own. Personally, I'm totally feeling the Robin Hood one http://www.pinupgirlclothing.com/seadhaco27.html
I'm new here, and not totally sure about pictures, so let me know if it's too large!