Unfortunately, people sometimes don’t want to admit that the school didn’t accept them because of their lack of tznius, so they will come up with other reasons, claiming that the school is prejudiced against them because of x,y, or z, without acknowledging that it is because of their refusal to abide by the school’s rules.
The parents or the children? I get the children not following the rules, but I’m uncertain why the parents have rules (not related to their conduct in school). Your example of covering hair: If her Rov said she doesn’t have to (my understanding is that is not uncommon), what does the school care?