I know it's probably not a good idea to say this on the internet, but I am a teenage girl. A lot of the stuff my classmates wear is inappropriate, and I don't just mean the girls. Most of the guys in my school have their pants riding so low that you can almost see the leg-holes of their underpants. None of my friends dress like that (We're all khaki and T-shirt types with a few jeans/buttondown types and a semi-goth thrown in for spice), but it's a hard act to keep up. I stopped wearing jeans a while ago and moved to khakis because I simply couldn't find any that fit me properly (It's hard to do martial arts when your pants are tighter than a dwarf's purse). I'm ordering all my back-to-school clothes from magazines instead of buying them in the mall, and not everyone's up to that. Some kids dress that way because it's hard to find other clothes easily.
Personally, I blame the over-sexualization of american pop-culture: It's a vicious cycle. Stars looking for shock value cut the cloth smaller and smaller, women (and men) in advertisements show more and more skin and are posed less and less appropriately in an attempt to sell products unrelated to clothing, and clothing manufacturers get the feeling that everyone wants to dress like these highly publicized people. Then the clothes no longer have as much shock value, so the advertisers and celebrities step it up.
Sorry about the rant... It's just when you work as a councilor-in-training at a girl scout camp and see an eight year old girl wearing a string bikini and boasting about being the only girl in the camp with a tube-top... and then find out that four other girls also have tube-tops? I'm sorry, but even I, a teenager, don't think it's right.