One really big thing that would help that I've only seen one person in my immediate circle ever talk about, is to start designing schools that are more difficult to shoot up. In the late 1950's, engineers and architects all got together to reduce school fires. I want to say 1957 or 1958. We started building schools out of materials that don't burn as easily. We put fire extinguishers in every class. We started having more fire doors, etc. Ever since then, there have been many years where no kids have died in school fires, and I don't think there's been a year where we've lost more than 10.
Do I know what this school should look like? No. It's possible to build this stuff, though. And it'll cost money, for sure. But so will every other solution. This is definitely worth looking into. Court houses and prisons don't get shot up. They have 1 or maybe 2 points of entrance/exit. They have metal detectors and other physical countermeasures in place. There are absolutely physical barriers and design formats that can make it more difficult for schools to get shot up by evil and sick people.
Do I like the idea of sending kids to places that look even more like prisons or institutions than schools already do? No. But maybe something could be done with design to make them look less oppressive. And I'd rather kids go to an ugly school and be safe than have them go to a better looking school and actually not be safe.