Sunday school is not a public institution, which is why it gets a pass. Similarly private schools are free to do this all week long.
I think even this supreme Court would rule the correct way. I wouldn't be surprised if it were even unanimous, but at worst I'd expect the 6/3 split with Thomas, Goraych, and Alito. There's only so far they can go when the Constitution was very blatantly clear on this matter.