Okay, but we’re talking about grade schoolers and, again, it has never been necessary for public schools to teach about the Bible in the past to give kids an education in American history.
So why you’re trying to justify Walters’ decision I don’t know.
Also, you can teach about Christianity without teaching about the Bible beyond generalities and without venerating it as a sacred document that was also primary founding document for the nation. Which it simply wasn’t. I’m not sure why the First Amendment alone doesn’t make that clear to you.