Notice that I did not say teacher or student. But there are certain teachers that I would want to be armed at all times. In one of my posts, I mentioned a fried that would check traps before school, carrying a handgun and then leave it in his locker during the school day. That same kid grew up and is now a teacher in that same school. He is also a volunteer fireman in the township he lives in, works closely with law enforcement as he is also a member of the villages city council. I would trust him MORE than most LEO's to carry a firearm in school. As a matter of fact, when I was on our School Board, I seriously looked into what it would take to allow him to carry. (There were laws against it).
There are many of us that carry every day without incident. As Drew said in his post, it keeps a lot of us out of trouble as we are aware of the power to take life that we possess.
Also the fact of the matter is that we protect banks, stores, congress critters, office buildings, etc. with armed people, but for schools we posts signs that says "This is a Gun Free Zone." and expect that to protect our kids. That is insane.
You are realistically looking at two options. 1) Rent-a-cop is armed and trained well and is posted at the entrance or 2) having a teacher that is qualified enough to handle a firearm. The problem with option 1 is that Rent-a-cop is an extra $50,000 a year from your school budget for something that is realistically not going to be used much (or hopefully ever). The problem with #2 is that the teacher's primary job is to teach, so while he could be of help once the shooter starts roving the hallways, the shooter could have sprayed three rooms of students before the armed teacher gets there.
I get the idea, and there are certainly a couple of teachers that I might trust to handle the situation correctly, but for example, my kid's elementary school (at that time) had literally 12 teachers total, and 8 were female aged 26 or younger. While I might be painting a broad brush, I'm fairly certain that not a single one would be competent to handle a gun. And the turnover rate in schools is decently high. You don't want to go to the trouble of training a teacher to be prepared for this kind of situation, only for that teacher to leave at the end of the year to go to a different school.
We all like to think that "well, if I were in this situation, it would have turned out differently because of X, Y, and Z." And to be honest, if you specifically had been there, then absolutely it might have been diffused. But you are basing that on YOUR training, and YOUR experiences. If you instituted an across-the-board directive that an armed guard is present at every school, you are going to get some highly qualified individuals who probably have equal to the skills that you possess. But you are also going to get some who are.....less. And the more financially strapped a school is, the more bottom-of-the-barrel of guard they are going to get. Some kid as a prank slips a roofie into his morning coffee, and suddenly you have an unsecured weapon that anyone can pull from his holster. Or maybe you get a security guard with racial issues and lousy trigger discipline.
You are looking at this from a best case scenario perspective of a former LEO or military becoming a guard. I am sadly looking from the opposite side, an institution that already struggles with getting its teachers basic school supplies suddenly has to fork out more money for a guard and gets the cheapest thing they can get. I just think that the odds of getting a crappy guard that results in an accidental discharge are higher than the odds that a homicidal nutbag is walking through the door.