Except that there is no "separation" in this case, is there? The school district seems to be advancing one religious belief.
Ahh, but without separation of church and state, the school could have told the complainers to go stuff themselves instead of taking the posters down, which they did.
In terms of the proselytizing, if that’s going on, then a concerned parent or other person with standing should sue.
Rights don’t defend themselves; they gotta be sued on.
Again, thanks to the atheists for that.