Wow,
There is alot going on with little girls. I remember being a sixth grader and feeling the social pressures.
I read that the problems jumped from make up to boys.
First, try to be very kind when discussing these girls with your daughter. Use friendly words to describe them. They are important to your daughter and you see the issue with their behaviors not the girls themselves.
All moms are at some point accused of not being understanding, I used to call my mom old fashioned. She wouldnt let me dye my hair. Well now Im an adult and have naturally beautiful hair.
Anyhow, these girls are treading new waters. They are noticing boys, attempting to get their attention(but boys that age are not there yet and it is frustrating or confusing).
Confidence is the tool to establishing her self. With self confidence she can ask the girls to leave her alone about the makeup and hair color. Helping her find her confidence is important and if these girls are hurting her self image unrelentingly, then maybe she can find new friends in any of her many interests.
Living in a small community offers more challenges, but it is still possible to find other mothers who believe as you do. Be careful to stay open eared and open minded with your daughter and her choice of friends. Without any wiggle room she may just become disobedient.