Appropriate Dress

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.

Kids think we don't know any better! LOL! Been there done that! I don't know the cost but have you considered hiring a stylist to show her how good she can look in appropriated sized and style clothing for her age. It might be worth it!
But you still are her mom and she still lives in your house and you say what she can and can not wear out of the house. She probably will still wear low rise but at least she can wear appropriately sized low rise (they really don't sell many pants that aren't low rise).

Okay well i've learned that mothers cannot speak to there teenagers hoping that they understand your way of thinking. So show her, first take a picture of her at her best in spontaneous moments then keep a journal
For her.
HERBAL LIFE, the nutritious way to lose weight and feel good doing it. Look it up for more info, anyone can have it skinny, overweight or the average frame, I would suggesT it to her so that she feels good about herself.
It sounds as if she is selfconcious and is acting out to undermined you cause your constantly telling her what not to wear, so show her there is a way to feel good, maybe both of you cAn do this change of eating with herbal life and will bring you closer, cause it works, believe me. That's the only way you' ll get her to feel better and hopefully she can wear the clothes she desires in a year,

This is a tough one. My suggestion is to take her shopping
somewhere that has a personal shopper. Most large department stores have "stylists" that can help. Leave her in a professionals hand and remove yourself from the situation. Think about the tv show "What Not to Wear". Clearly she cannot, or simply won't, listen to your pleas. The one thing you need to be really careful about is her remarkable sense of confidence. For any girl who is larger than her size 2 friends will struggle with size appropriate fashions. Sounds to me that you have a confident, self reliant and self assured daughter. These are not traits to be discouraged. Peer pressure will ultimately take care of this. Maybe a little reverse psychology will work. Tell her how great she looks, maybe buy youself an outfit that looks/fits just like her so you can be "twins". It's just a thought. Good luck.