They’re called air marshalls and they are put there by TSA. This article says being an air marshall might be the hardest, most stressful job ever devised. Everyone knows flying is stressful. It’s almost like a form of solitary confinement. Electronic diversions and moderate use of alcohol and over-the-counter substances can help. Air marshalls are not allowed to do any of these things. They just have to sit there. They have high rates of suicide compared to other law enforcement and military jobs, and they also die of heart attacks, strokes and blood clots.

So I feel bad for the air marshalls after reading this. I also wonder if having a stressed person with a gun on the plane makes me safer statistically than not having one. I feel the same way about armed teachers. There is just a certain rate of mental illness and substance abuse prevalent in the human species, and having more guns around means that more people will have access to deadly weapons at times when they are not thinking and acting clearly. You have to compare that to the very rare (but horrifying) rate of homicidal maniacs walking into schools and shooting. And teachers are not trained in use of deadly weapons or screened for mental health to the extent that law enforcement officers are, at least to my knowledge.