There were “Blue Laws” in CT with regards to the selling of liquor until very recently. These laws might have originally had religious motivations, but that’s a perfect example of something that isn’t a violation of the First Amendment. The law can be motivated by religion without establishing a religion.
Making a public school explicitly Christian is slam dunk establishing a religion. I don’t get how that’s even being questioned.
For those talking about the good old days when the Constitution wasn’t enforced and it was just the Wild Wild West (literally), that’s not a good proof that it’s OK. They were just doing a bad job of being a Constitutional Republic back then. Don’t romanticize it.