There are always going to be fights about what gets taught in schools, and what isn't. It's an inevitable consequence of government run schools. I don't agree that it is a free speech issue.
Children do not have the full set of adult rights.
No need to emphasize "government" -- It's an inevitable consequence of schools.
Private secular schools have fights about what gets taught.
Private religious schools have fights about what gets taught.
Children have more rights in public schools than they have in private schools. Tinker v. Des Moines: "It can hardly be argued that either students or teachers shed their constitutional rights to freedom of speech or expression at the schoolhouse gate."
There are truancy laws. Private schools can have speech rules, but parents are free to choose such schools, and so free speech rights are not an issue.
Children do not have the full set of adult rights.