D.A.
My suggestion is to go to a PTA meeting and bring up the issue with the Principal and other parents. I was a teacher at a middle school for 4 years and could not believe how some of the kids dressed. I made the girls put their jacket on to cover their cleavage or their butts. I told them that any body could see their breast when they bent down over their desk just to write, or if they walked by them. I also made them wrap their jacket around their waste to cover up the butt when they sat down.. The boys were told to pull up their pants. I didn't want to see their underwear.
The principal has a big say in this and can enforce the dress code. Dress is considered a distraction, a major distraction in some cases. We had a lax principal that let the boys hang their pants down, the girls wear pajamas and slippers. It was ridiculous. Finally we got one that enforced a dress code (somewhat). The teachers were allowed to send students that were inappropriately dressed to the office if they didn't cover up or comply. He even told the boys that if they didn't wear a belt and pull up their pants, he had a piece of rope he would tie them up with. Parents were required to sign the dress code policy and abide by it. If the students didn't abide, they could be sent home. The district was considering a uniform and it would have been a good idea. I have so much sympathy for you as a parent of a female. I was so worried about these girls , the way they were dressing , and about what thoughts they were inviting from the boys. Unfortunately the media has encouraged it; videos, fashion etc, and so the girls think it is okay. Is it any wonder the teen pregnancy rate is so high?!
Seriously though, parents have a lot of power regarding such policies. You need a cooperative principal. But you have to speak up. If the principal isn't cooperative, go straight to the district, even the superintendant. Something has to change.