Do you have many mentally unstable students in your classes?
It's not like all mentally unstable students wear signs identifying them as such; for some it's very apparent that something is amiss, and for others it's far more insidious. The onset for psychiatric disorders such as schizophrenia is often in the late adolescent years for males, a bit older for females. People who'd previously been psychologically sound can begin to experience symptoms as the disorder manifests, some of which aren't always immediately obvious. Even if a student does have a history of psychiatric problems, privacy issues often disable professors from being made aware of it, so unless they're exhibited in class it's very possible they are oblivious. James Holmes had been a PhD candidate in neuroscience with a sizable grant from the NIH, a prestigious internship to his credit, and was from a loving family and idyllic community. Though he had undergone psychiatric treatment, many were probably not aware of that, and his age and lack of a criminal history enabled him to legally purchase all weapons used in the Aurora massacre.
My grandmother was a professor for years, first at a major state university (Virginia Tech) where most of the students were traditional age, and later on when she didn't take to retirement she began teaching a community college near where they relocated. She actually encountered more problems with older students at the community college, some of whom appeared to be mature, functional members of society. A veteran in his 40s became enraged that he was doing poorly, even though she'd invested time in helping to guide him in overcoming errors that he continued to make, and screamed at her in her office, then stalked her. He followed her to the house of her elderly mother and sat outside in his truck. Some of the livid students are ones the professor has had minimal interaction with previously. A woman who was in her late 20s or thereabouts was angry that she'd been withdrawn from my grandmother's class for failing to attend it, and was consequentially losing this grant money she was receiving to be a student. She came and confronted my nana after a class, screaming at her, and pushing her off the stool where she'd been sitting. A previous back injury had already caused extensive damage, and the fall ruptured more discs in her back and pinched nerves. She had to start using a walker. Thank God neither brandished weapons, though. (And carrying one herself wouldn't have been feasible, since she couldn't even lift a gallon of milk or wear a thick coat due to her back problems, and had to keep her purse extremely lightweight).
My grandmother taught at Virginia Tech during the 2007 shooting, though was on sabbatical at the time. She said what many faculty members, students, and parents wanted were not guns on campus, but a swifter and more organized emergency response. Students were notified by email, not text, more than two hours after the first shooting, which had falsely believed to be an isolated incident. Had the entire campus immediately been put on lockdown and everyone texted and told to shelter in place, it's very likely the death toll would have been lower. But back then campus shootings weren't as tragically frequent as they are now. During the UCLA shooting last spring, texts were rapidly sent to everyone in the community telling them to seek shelter. As with VA Tech, though, there is a lack of rooms with lockable doors.
I write longggg posts when I can't sleep, hahaha. Apologies.