You keep running back to that as if there was some truth to it. But as a typical leftist, it's either your way or no way. We have suggested arming schools, making it much more difficult for assailants to enter schools, schools making sure doors are locked and nobody can get in, but no, it's either prohibit everybody from buying a weapon until the age of 21 or nothing at all. And anybody who disagrees with you must want to see school shootings continue.
The method to your argument (whether you know it or not) is what used to be known as reverse psychology; the act of trying to make others feel guilty by not agreeing with you. It didn't work back in the 70s and it's not working now. People have different opinions on how to solve or reduce this problem and none of them are flawless including yours.
I've heard suggestions from experts like man traps. A man trap is two doors spaced about eight feet from each other. You can get through the first door but not the second without a person monitoring the door to buzz you in. If something seems amiss, the first door locks and you can't get through the second so you have nowhere to go until the police arrive. Another suggestion made is have school wood shops cut wedges for each classroom. In an emergency, the teacher can wedge the door closed so even if the attacker tries to shoot out the door handle or locks, he'll be helpless against a wedged door. It's an extremely simple and very inexpensive solution.
I (and others) just don't believe in punishing everybody for the act of one or so persons. It's like taking drivers licenses away from teens if one of them gets drunk and kills somebody with their car. It's not a solution because some teen somewhere will find a way to get their hands on a car, get tanked up and wreck the vehicle possibly killing other people. Our solutions or that of experts are not flawless either, but at least we're not punishing everybody to try and solve the problem.