I also think that the ultra-orthodox are full of crap with their medieval beliefs bordering on superstition. However, this is a private school and they are entitled to make their own rules. If the child and the mom don't like it then use the public schools or find another private school with less stringent requirements or try home-schooling.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: Ultra-Orthodox school bars girl over her 'mother's immodesty'