One of the issues I have always had with these types of incidents is that of architecture. Most schools, especially those built decades ago, are not tactically defensible. It’s a shame we have to consider this, but here we are.
Most public schools are, as one police officer I worked with said, “a shooting gallery.” There are long straight hallways with little to no cover anywhere. Classrooms have one entry and exit point. Windows do not have an option to be opened. Locking down in a classroom is better than nothing, but if an attacker is determined to get in, s/he will. And then everyone in the classroom is a fish in a barrel. Classrooms should have doors adjoining other classrooms, providing alternate exit/escape routes. Windows should be able to be opened in an emergency enough for students and teacher to fit through. Second or higher floor classrooms should have escape ladders in addition to the emergency windows.
At the very least, each floor of a school building should have a designated, hardened panic room purpose built to protect those inside and keep intruders out reliably. Perhaps adding an emergency escape route (ie, underground tunnel for ground floor) could be installed as well.
Hallways should have at least one or more turns or other hallways that cross long hallways, providing a way out of the line of fire. Long straight hallways only make it easier for a single shooter to take easy shots at those clustered in the hallway.
Cameras should be emplaced, and a dedicated person should be in a protected room to monitor them, and if necessary, guide staff and students to safety, or act as realtime intelligence for law enforcement when they arrive.
Finally, schools need to STOP training students and staff to lock the door and hide, and offer no resistance. This is a recipe for lots of people to get dead and grievously wounded. History has shown that active shooters do not expect resistance, and when resistance is offered, move on to other targets that do not resist. Thus the training should be to seek an exit and go at the first sign of trouble. If a quick exit is not feasible, resist with everything you have, because it might save lives.
The Army teaches OCOKA: Observation, Cover/Concealment, Obstacles, Key Terrain, and Avenues of Approach. Again, it sucks to have to think tactically about school architecture, but it seems the time is now (actually a long time ago). Everyone says we need to protect children, but no one wants to foot the cost to do so. To anyone who is a parent: How much is your kid’s life worth?