I have no solutions. There are more guns in America in circulation than anywhere else in the world. Furthermore, when the riots began, gun sales grew month after month. Restricting what is already in circulation seems implausible.
I will say that I used to go to schools, do site evaluations and propose security solutions that would help mitigate the damage of an active shooter. And, I would always try to find a side door to enter to show them how easily I was able to get in without them knowing.
Sans very few, it always comes down to the budget. It's not difficult to design and implement a solution that funnels people through specific openings where staff can have more control over who comes and goes from the campus. But, the budget. We're not talking millions of dollars, either.
More often than not in my world, which is no longer in schools but still holds true most everywhere in the security world - the decision-maker seldom puts a value on prevention and when something happens, they magically come up with the funds.
Kids don't have to go to schools and it feel like a prison. But at some point, the school will have to spend money to bring up the level of security. They need to write and enforce policy. You have to approach it like your school is next because if you play the law of averages, they won't do but a little more than they did the day before.