It's my opinion that it's not for you, or I, to decide what's best for anyone else's children. It has always been the guardian's responsibility to raise their children NOT the school's.
Then there's the fact that the Bible, they ALL agree should be mandatory, is full of horrorific stories like no other and no one is saying it should be removed or that telling small children their souls will burn in hell in a lake of fire and brimstone if they don't do what they're told should be banned.
That's censorship, it's forcing one religion on EVERY child, violates the constitution and it's also hypocrisy.