This is the big issue, preventing mentally unstable people access. It's difficult also as someone who is stable today may not be tomorrow due to some circustance or other. A tough issue.
Active shooter drills...Hmm, that's a sad state of affairs and not something we have here. Unpleasant for parents having to worry about such things for their kids. I don't know what the answer is. Take the guns. A car gets rammed into a school yard. Take cars away. Stabbings. Ban knives. Clearly that's not possible of course. Educate humans though? Hmm, now there's an idea. Help those with mental health issues, deal with social issues, global issues? More ideas that could be considered. But...humans are good at killing and have always found a way.
Firearms are an easy target and they can, and have, done some terrible damage for sure. Same as drugs and alcohol (those under the influence) Same as rapists, politicians and shoddy engineering and building standards. Hazards are everywhere.
A little more education may help.
It's a really complicated issue and one that we probably won't find a solution for anytime soon.