I'm a teacher on a college campus. I can imagine exactly what would happen if the school decided I should have a gun in the classroom.
First, I can easily pass a background check. I have no criminal record and no history of mental illness. Buying me a gun shouldn't present any problems. (But this may not be true for all teachers at all schools.)
Next, I'll just assume that the school pays to provide me with proper training, and that I do reasonably well in target practice. The cost of guns and training for all teachers, as well as the likely increase in insurance premiums, may be prohibitive for an already cash-strapped institution. But for this story, let's pretend that the school can afford this without having to cut back on books and supplies or fire some of the maintenance workers.
Now I have a gun at school. Where will I keep it? A locked cabinet would be a safe place, but it wouldn't do me much good if a gunman burst into the room. I can't keep the gun in a desk drawer or in my book bag, since it would be too easy for a mischievous or malevolent person to get it. So I guess I'll have to wear it in a holster on my hip. Will my new security-guard look support the comfortable, nurturing relationship I have with my students? Hard to say.
Next let's imagine that the worst happens and some sick loser decides to shoot up the school. If he bursts into my classroom, guns at the ready, it is unlikely I will have time to draw my weapon. He will get the first shot, and that will be that. In fact, this was the first objection my students raised when I asked them if they would feel safer if I had a gun. They pointed out that a killer who knew that teachers were armed would intentionally target me first. That wasn't a comforting thought for any of us.
In a slightly different scenario, the killer starts shooting in the classroom next to mine, and I'm aware of what is happening. I might try to help the occupants of that room by running over there with my gun. But I probably won't be very effective. I don't know exactly where the shooter is in the room -- and I don't want to accidentally shoot a student -- so I'll need at least a tiny bit of time to find my target and aim. At the same time, I'm coming through the doorway, which makes me a nicely framed target for a guy who already has his guns out and doesn't share my concern about collateral damage.
Maybe my students and I can just crouch behind our desks and wait. I'll aim my gun at the door, ready to take out the shooter as he enters. At this point, the question might be, do I have the heart to pull the trigger? I do. But I had better be a really good shot. I'll probably have just one chance to bring this guy down. I'll only have time for one shot. If I miss, that's it. My handgun against his semi-automatic rifle is no contest.
At this point, some people might suggest that having even more guns in the classroom would solve the problem. What if some, or all, of my students were armed? Couldn't they simply overwhelm the gunman with their collective firepower?
Maybe. And in that tiny classroom, in an atmosphere of terror and chaos, there will be any number of stray bullets, ricochets, and weapons pointed the wrong way. How many of us will be caught in the crossfire? I don't want to find out.
Don't forget, by now the police have been called and a SWAT team is in the building. If we are all waving guns around, how will the officers know the difference between good guys and bad guys? How likely is it that an innocent person who happens to be holding a gun (and is too terrified and excited to realize it's time to drop it) will be perceived as a threat and be shot by our rescuers?
In the meantime, think about those armed students. Most are between 17 and 24 years old. Science tells us that most of them have brains that have not yet fully matured, particularly in the areas which are needed for rational decision making. How helpful will they really be in an emergency? In the course of an ordinary school year, what will happen to all those guns? Some students will lose them, the same way they lose their cell phones and their textbooks. Some will occasionally leave the gun at home, sitting on an end table or perhaps cleverly hidden under the mattress, where their six-year-old siblings will find it. Some will show off their guns to each other at lunch time, and now and then someone will be accidentally shot.
And, while the overwhelming majority of my students are good-natured and peaceful, it is inevitable that there will be the occasional troubled soul among them. Someone who will experience road rage on the way to school. Someone who will be obsessed with jealousy and hatred for his romantic rival. Someone who will become depressed and suicidal. Someone who will go nuts because he failed his classes. Someone who will take advantage of the opportunity to become the next school shooter.