A hard one. I would not send her to school and take TV and magazines out of the house. It is such a brainwash the female image with no hair. It is a shame. If not messed up with, it will pass. The hair will fall and thin out later. If you pull out of bleach it will strengthen the hair.
Really, the problem is the influences. She couldn't think of it herself. Ask if someone at school laughed at her and said anything and help her see that it just isn't true and has nothing to do with being a girl.