I don't know if I should tell my friend?

My friend.... quite frankly is a bit of a slut. But, I don't really know how to tell her? See, at my school, you have to wear stuff thats BELOW the knee. But she, she doesn't seem to get that. To her, it's like one centimeter under her panties, barely covering her AHEM. She seems to like attention from guys, but lately, I've been getting attention and have been named hottest girl in the cl. But she still acts slutty and likes to wear the shortest skirts and shorts, the tightest and lowest cut shirts she can manage to find, last year, everybody used to tell her to HER FACE that she was a slut and ask her if she had heard of pants or below the knee or NORMAL shirts. But yeah, I don't know what to tell her.