An AR15 is not at all a "high-powered weapon," it's just an intermediate-caliber semi-automatic rifle. Again, firearms fitting this general description have been on the civilian market for over 100 years, and long before the AR15 became affordable, other rifles filled that same niche. So why indeed have school shootings become prevalent? It's not the availability of rifles, no matter how much you imagine that to be the root cause.
The funny thing about firearms is that they create an equality of sorts between overweight, middle-aged women and young aggressive males. And I am not saying teachers should be required to carry, only not forbidden from carrying. This doesn't make them a guaranteed source of anything except risk to would-be wrongdoers. School shootings seem to be the domain of one or two aggressors, not gangs. And it is precisely the victim of gang violence who benefits from the higher magazine capacity and faster firearm operation which seems to scare you about semi-autos.
You present scenarios based on fear and ignorance combined with blind faith in legislative solutions handed down from on high. I argue based on the principles of self-ownership, personal responsibility, and individual rights. Again, I ask you: if I decline to obey an edict, yet harm no one else, why should I be deemed a "criminal?" Do you really advocate violence against me because I dissent and disobey?
Semantics of firearm specifics aside, the point was that someone with the reflexes of a young man with a weapon that can fire rapidly against someone who is stressed, underpaid and armed with a pistol is not going to be an equal battle... and therefore likely not a deterrent.
If the availability of the weaponry is not the root cause, then why does the US have so many more school shootings than other developed countries? Other countries have all the problems you listed previously, but the USA has so many more school shootings. If not the ease in obtaining the weaponry then what is it?
I answered your question in length already. Your question is too simplistic and doesn't take into consideration the wider societal affects. What works for one individual doesn't necessarily work for every single person within a society. You might be able to handle Fentanyl perfectly safely, but that doesn't stop it being a societal issue. That aside, the changes I'd like to see probably wouldn't affect you at all unless you have convictions for domestic violence or known mental health issues related to violence.
How many school children are you willing to witness being murdered in their classrooms before you're able to look at solutions that have been successful in other countries?
The USA is a massive outlier in school shootings. This problem has been solved in numerous other countries.
I'll take experience of age and training over unfocused youthful aggression any day. Your refusal to accept the deterrent effect of increased risk of effective resistance is irrational. It takes very little consistent training to be reasonably proficient, and it wouldn't just be the teachers. Janitors, the principal, clerical staff, the lunch ladies, librarians, everyone has the natural right to self-defense with the most effective technology they wish to use. Disarmament is trespass.
If the availability of weapons is the root cause of violence, why is it only manifesting in mass shootings at "gun-free zones" while violent crime rates overall plummeted, and most violence is associated with black market drug trade and economic segregation? Do you know what else started spiking in the 1990s? Drug prescriptions for kids. Antidepressants, ADHD medication, and other psych meds meant to help them cope with the antisocial school structure imposed upon them. You look at the guns as the root problem. I suggest we look at the schools themselves.
Your answer of government violence is too simplistic. How many people are you willing to see murdered to impose your dystopian ideal? We already see brutal police abuse against people who are making, selling, and consuming drugs in spite of prohibition. Alcohol prohibition created violent gangs in the 1920s. Government violence is not the answer, it is usually in fact the root problem of societal ills.