I am quite unconvinced by the "but its an opportunity for a discussion" argument. Of course, most /books can be an opportunity for a discussion, and it is something some parents (perhaps mainly of girls) will think of doing but that's not the point is it?
Children growing up today don't need us to have 'discussion' to tell them that boys can be courageous, active and save the day. These stories are told a million different times in a childhood - through toys, adverts, books, films, tv, role models, etc. Combating the comparitive dearth of messages about female agency with a simple "girls can do that stuff too" just isn't enough, children will make their own conclusions from the world we provide them with, whatever we say. I don't think it's unreasonable for schools to have a role in challenging society's sterotypes be they of gender, race, sexuality, disability, class etc by providing materials that reflect equality.
A while back, on a previous (not the current) thread about films with positive messages for very young girls, the OP had an interesting requirement. She didn't just want films which passed the Bechdel test, or had a fiesty heroine. She was looking for films which instead of having a women or girl having to struggle against societal expectations (say like Brave or Mulan), just assumed girls had equality in the first place. From memory I think she also wanted films that weren't focused on a romance.
Its amazing how few films suitable for young children fitted those requirements. Kiki's Delivery Service and Lilo and Stitch are the only two that come to my mind now.
Its not that there isn't a place for stories about girls and women striving to be treated as people, stories like Mulan and Brave can be inspiring. But they still reinforce that a woman having agency is unusual, something that needs to be fought for, the heroine is often subject to sexism in the film and that the prevailing view in their world is that girls aren't able to do heroic stuff.
Why can't small girls just take it for granted that they can have adventures and be treated like equal people? Rather than have most of the stories they read either deny them agency, make them a prize or make a big deal about how much they need to fight to be taken as seriously as boys and men are.
Good on you OP. I think that schools ought to take equality seriously. Its not censorship to be fed up of children being fed the same old sterotypes and wondering why a school is feeding children this. How many parents would have read this to their children without 'a discussion'.
I can't guarantee that they won't think you are a bit crazy, but I think its worth the risk if it means that the school thinks more carefully about the materials it buys for children.