Having "a good guy with a gun" as the NRA puts it, especially in this situation, will not automatically make people safer. I doubt anybody who thinks that having adults armed would deescalate the situation has probably never been in that position before. In the moment, you're not going to "save the day" and more bullets flying through the air poses a greater risk to everyone else in the area than one threat.
Not to mention having a society where even your teacher has a fully-loaded gun on them at all times for "self-defense" is a great way to foster authoritarianism and fear among citizens.
