comparing the two situations

In one we have one guy, armed with a revolver going after a specific target surrounded by a group of people trained and on guard for just this sort of thing happening.

On the other we have one or two guys often armed with more powerful fire arms then a revolver and/or more then one weapon going after random targets. In the case of a school, there is generally only one teacher per classroom and unlike the Reagan shooting, school personal are spread out from the potential targets (indeed , they are among the potential targets). Also teachers are not going to be "on guard" like the secret service is as not only do they have other primary responsibilities (since their job is to teach, not be a body guard) but statistically such shootings are still rare. Unusually common compared to other countries but out of the 300 million of us, very few of us will ever be directly involved in a school shooting. The mind set of a secret service agent and a teacher is probably very different when at work.

So no these two things are not directly comparable. If anything it shows that the "arming the teacher's" crowd have an even weaker argument since if guns would have protected anyone, it would have been reagan. If you want to say that guns can protect from other guns, this situation was set up for that.

And still Reagan got shot. And it wasn't the fact that his body guards had guns that stopped Hinckly (who fired 6 shots in less then 2 seconds). The people who first responded didn't even have guns themselves. Hinckly was punched and tackled to the ground. (not to mention how the secret service firing at Hinckly would have meant also firing into a crowd)

So no the Reagan shooting and most mass shootings are not directly comparable since in the latter, there is even less reason to think a "good guy" holding a gun would help. (and indeed in cases where someone else did have a gun, they don't seem to have helped until after the shooting was already pretty much over) But the argument is valid.

In most cases of mass shootings (and shootings in general) there are fewer factors favoring the "good guys" taking out the bad guys and more against that happening.

Yes guns can be useful in some situations. The Secret Service does carry them for a reason. As does the police and soldiers. You wouldn't want to be unarmed in a war zone.

But guns have far less utility then those proposing more people being armed seem to want to admit. Most of us aren't in a war zone. Very few of us go through our day thinking they have to watch that strange person who just walked by in case he needs to be shot. In many situations guns are at best a safety blanket. At worst part of the problem.

Indeed, considering how easy it is already for potential shooters to acquire guns legally and stay under the radar until it's too late, making all sorts of guns even more accessible would seem counter productive.