Unless her legs are so hairy that they cause understandable embarrassment to her, I certainly don't think that shaving should be encouraged. It's also inappropriate for a 12-year-old girl to be wearing make-up or clothing that mimics an entertainer like Ms. Spears. Trying to control your daughter in these realms is a delicate and difficult task because she is convinced that she must do these things in order to be thought of as desirable. No doubt her hormones are also kicking in and this accentuates -- and causes -- many of her extreme mood swings.
Letting her know that you understand why she feels pulled toward looking and behaving older is very important. Even though she will loudly disagree with you and call you unfair, she will understand why you're imposing limits and that you do know what's driving her to behave this way. Every adolescent tries to guilt her parents into granting her requests by invoking the "but everyone in my grade does this" plea. I guarantee you that the majority of sixth-grade girls are not wearing make-up and sexy attire to school or shaving their legs. Ask a few moms of girls your daughter's age how they're handling these problems. You'll also benefit from reading Parent/Teen Breakthrough: The Relationship Approach, by Kirshenbaum and Foster.