This shooter wasn’t about God being in the schools or not. He intended to kill kids of that particular school......and wasn’t about to be stopped by any from his intentions short of getting shot himself.
When we speak of God being in schools we need to also speak of how. As a byproduct of general believed evangelism is the only valid how. And that could in some situations have mitigated the damage. Even in the case you spoke of, there was a single face-off with police, not a mass murder.