- Make any modification that allows full auto illegal (e.g., the bump stocks that fell through the cracks).
- Ban/restrict armor piercing, tracers, and similar forms of unnecessary civilian ammunition.
- Strictly enforce existing laws. You might be surprised how many there are. See this piece from The Hill.
- Spend some money on proper background checks and extend the wait time between purchase and possession if necessary. All vendors require a license and any personal sales must go through a licensed firearm vendor. No more gun show purchases.
School (General recommendations, specific recommendations require proper site surveys)
- Fencing (fence, walls, etc.) with maximum stand-off distance and appropriate sensors
- Gated entrances with armed security through which visitors can be checked and provide ID before they enter the space.
- Metal detectors and bag checks at all entrances.
- Staggered schedules, multiple entrances so that students aren't bunched up
- Architectural design features that allow for shelter, evasion, and escape.
- Emergency plans that are regularly tested.
- Uniforms & badges for students and staff
- Once in school, stay in school until the end of the day.
- Strictly enforced discipline.
There's more, but you get the idea. Local jurisdictions can make many of these changes without the feds getting involved. So what are you waiting for?
You're not going to ban assault weapons - they're already illegal because assault weapons have full automatic firing capability. The semi-automatic AR-15 style weapon is not an assault weapon. It is the same as most hunting weapons - it just looks different. If you try to take them away, you'll start a nasty revolution. If you want to buy 14 million of them back, where will you get the money? Finally, recognize you won't get them all back anyway.
Weapons in schools. Tough call. Recognize that few people are exceptionally accurate shooters even when calm and will become decidedly less accurate under pressure of an actual event. Who will take responsibility for the inevitable mistakes and subsequent litigation or will they get a standing waiver from all legal and civil responsibility? I think it should be optional. If the teacher wants to assume responsibility and the school allows it, they should be allowed to carry after completing a rigorous training course and then attending regular, intensive practice sessions.
Guns did not get us to this point where people start indiscriminately shooting at others. That is the "mental" part of the problem. Morals, values, and individual/group identity are intertwined in the problem. The fix is possible but no matter what I write here, will likely not happen absent a few more rounds being fired. People are not quite fed up enough no matter what the press and social media would lead you to believe. If they were, the changes I outlined above would have already happened. So far, it's lip service and politics.