After the Croatian war ended in 1995, a perfect storm of conditions led to a huge rise in violent bank robberies. You had a country with a very low rate of gun ownership that just went through a war. A lot of people simply never returned their weapons after the war. Obviously, those people knew how to use them, too, having had 4 years of practice. Those same people also sometimes had PTSD. The country was poor, but undergoing that awkward transition from command to market economy, with everything suddenly being available, but there being little money for most people to buy it. There was also a generation of young me in their early 20s who were drafted straight out of high school with little education and even fewer skills to live in a civilian society. Remember that this being a massive war of independence, it wasn't like decommissioned soldiers were some small segment of the society. Most young men saw military service in some shape or form.
So naturally, some of these young men started robbing banks. And not with a not they passed to a teller while having a bulge under their coat that my or may not be a handgun. These guys would show up with assault rifles, fire a few warning rounds in the air and be out of there before cops have a chance to show up. So the banks resorted to hiring private security. I remember them well. Massively built guys in their 20s with a red beret and military-looking fatigues standing in the vestibule, with a 9mm in a holster. Scary looking guys, but pretty soon, the would-be robbers figured out that obviously, private security can't shoot first. They're there to stop shooters. So, of course, the decommissioned soldiers robbing banks started shooting these poor private security guys. Between 2003 and 2009, almost a dozen were killed. In two of these cases, banks had two security guards. In both cases, they were both killed. Fully-automatics, no warning, just walked in, emptied a few clips into these guys. In one case, a bank had no security guard, but 5 cops accidentally walked into the bank during the robbery. The single robber killed one and wounded the other 4 before escaping. How? Simple. The cops didn't walk in with their guns drawn because they weren't there because of the robbery. They had no idea, so the shooter shot first. By the time they responded, he had escaped, but not before shooting every one of them. AK-47. Never jams, never fails, as former soldiers liked to say when I was a kid.
You can have an armed guard at the school. You can have two. These shootings tend to be planned. The shooters will simply plan for killing the guards first. It's not that hard, since they can't fire first. Or you don't have to kill them. Mass shooters these days generally don't plan to survive. If the police officer is in their office, just go to a hallway on the other end of the school and shoot until they show up.
