I don't see a problem here ...
... because I KNOW (right?) that every schoolboard who proposes such a curricular application WILL LET ME -- as a parent -- sit in on the lessons during classroom time so that I can be fully privy to ALL that is being taught to my child.
No public school is going to teach my child in private away from the possibility of my open observation. Otherwise, they'd be imposing a cultified indoctrination, and since it's in my full interest as a parent to know exactly what my child is being plied with, the public sector of teachers wouldn't want to teach my child something behind my back.