There is so much clamor about "Gun Free Zones" (I know it's BS) that it skips over something I hadn't considered until I read it recently. Forgive my ignorance but it's hard to see what isn't there or in this case not put in front of you.
First, guns in any environment increase the risk of something happening. The same can be said for mousetraps and rakes, the presence of them greatly increases the chance someone steps on one.
Second, workplace shootings become far more likely or possible if guns are there. Weird trivia - in 1993 or 1994 more people died in Post Offices than in electric chairs (capital punishment). The USPS ended Guns in their workplaces shortly thereafter.
Lastly - here's the point that I'd simply missed previously- when there is an incident and the police have to respond the fact that guns are banned greatly decreases their risk. Yes, they are hyper aware that other guns may be present but banning guns in that environment means that it's highly unlikely that someone has a gun in their desk or stuck away somewhere and that they might come out guns a blazing trying to be a "good guy". In that situation police have to assess and react not knowing who is who - thy are just going to get the situation under control. The sloppy handling and poor marksmanship elements of the "good guy" comes into play too.
But if that was brought up in any of these discussions we'd have more of the story and that would not make for TV news drama.