Public schools are where children learn to socialize, learn about teamwork, and that not everyone looks or behaves the same way as they do.
Just because a kid is removed from a classroom for a particular subject, they will still discuss these things with their peers. Having missed out on the subject, they will not have a full understanding of what their peers are talking about. It isn't about whether the child agrees with the subject or not. It is providing them with the information they need to decide for themselves.
Children who proclaim that their parents know better than the teachers also learn a new skill called "self-defense". The ones that get picked on are the ones who, through no fault of their own, are perceived as different or weird.
Been there, did a lot of that.