Y'know I got hosed by going to high school in the early 90's. Flannel, loose jeans, I blame Grunge music dammit! lol
I would have to say school girl as well. My wife is only 5ft 95 Lbs she she can play the role very well so that helps!
Personally, I like the variations on the Naughty Schoolgril costumes better because I have great mental associations with that "look," and not-so-great associations with Teachers. Apologies to the girls who ARE teachers now, I'm sure you're hot, it's just that I'd rather you were dressing up in a nearly see-through dirty cheerleader outfit than the skirt-suit look, even if it's skimpy. But, that's just me.