If it forbids the hymn then it is teaching the God-centered students that religion must be hidden away like some form of bathroom activity. ( **Not** a neutral lesson.)
If they allow the hymn then the government is promoting and establishing a God-centered worldview that is offensive to those with a godless religious worldview. ( Also not a neutral action by the school.)
Solution: Begin the privatization of all K-12 schooling.
I thought you said that schools don't teach anything?