When you hear of a policeman seeing anything like you described and actually preventing a school shooting you let me know. Anywhere in the nation.So if a county deputy sheriff sees a person carrying an AK 47 and 500 rounds of ammo into a school, he can't act on that because the county is a 2A Sanctuary and the "no guns in schools" statute is an infringement of the 2A? Is that how it works? Now, a city policeman is not affected by the county ordinance because he is not a county employees so we have to call a city policeman to arrest the gunman, but once he arrests the gunman the county jailer won't take him into custody because he is a county official and the 2A sanctuary ordinance says he can't be involved in any infringements of the 2A. Sounds great! Where do we sign up?
And the city cop wont be doing squat either if he operates inside the county as the sheriff outranks him and will be tasked with arresting said city officer. And the ability to deputize all the help he may require.
But let's go with your red herring. Said cop sees person going into a school armed as you describe. Nothing prevents the cop from following and observing . And IF it becomes clear the armed person is launching an attack on the school the said cop is free to act.
That's assuming one cop would ever place himself in that position before calling an army and waiting until they show up long after the damage is done.
Should be plenty of places to sign up in a little while .