She should be allowed to wear whatever she's most comfortable in as long as she's not hurting anyone. She shouldn't be allowed to scratch people though, that's going too far.

I graduated 6 years ago. There were a lot of people who dressed as vampires in my school when I was in 9th and 10th grade. They all wore capes, fangs, and face paint. They never caused any trouble, they were all really nice. There was 1 girl who was really small and she carried a coffin-shaped purse, she was the cutest little thing and one of the sweetest people in the school.

When I was in 11th and 12th grade the school no longer allowed anyone to wear certain things. We were no longer allowed to wear an all or mostly black outfit. Justin once got in trouble for wearing dark blue jeans with a black shirt that had a Bible verse on the front. They said his jeans were too dark and looked almost black.

The year after I graduated the school got even more strict on their dress code. You had to wear shirts with sleeves. You could have no visible tattoos and visible piercing were limited. They even put a restriction on shoes. Kids had to wear their shirt tucked in with a belt, that was the rule, it was mandatory. It got way out of hand.