I like simple solutions that offer the best approach and greatest proven level of safety.
You do what we started to do when schools, back in the old days would catch fire.
We built schools to be fire proof. Used non flammable materials, practiced fire drills. Good communications with the authorities. Working Fire hoses on every floor and every section......and alarms.
School shootings are a different caliber of a disaster. But we know a lot more now then we did back ten or twenty years ago.
You start with fortifying ways to stop a car from driving into the school lobbies It's not expensive to put those concrete crash guards in front of school lobbies.
Cameras in all hallways with an immediate connection to the police so their swat teams or first responders can understand where the shooter is....... etc.
Bullet proof materials for all class room doors and locks that can be bolted shut from inside the classroom.
A way for each class to report to the school office or speak directly with the police.......... reporting where the shooter(s) is or heading to.
School teachers carrying guns......only those volunteers with former military or police training. Only those who will volunteer their off time to work with the police during the summer or holidays on how to keep their students safe. How to defend or blockade every thing that would be necessary to keep students secure including training in first aid and the equipment needed.
And finally it would not be a bad idea to offer the police a free school breakfast or lunch or afternoon snack and designated police only bathrooms. You see cops having a meal in the lunch room or cop cars in the parking lot it sends a message. We serve and protect.
Stop and think how it would have changed the attack if two or three police officers were in the school. If they were so familiar with the school that they would understand immediately, if they heard that a shooter was on such and such hallway and their actual location.