I've witnessed three NDs with M9s, one of them fatal. Another one wounded a 9-year-old girl standing nearby. All despite the safety catch.
I think I understand the military obsession with mechanical safeties, but I think they're unnecessary from a training standpoint. I think they add a moving part, which becomes a point of failure when dealing with Joe.
To me, going to the Glock would simplify training: the sole need would be to teach kids to keep the booger hook off the bang switch. You'd still get military NDs (because Joe will be Joe), but I don't see a downside. And even though I'm vocally (but cordially) anti-Glock in my own life, going over to that platform is a no-brainer in any institutional context.
They're just too easy to train on, maintain, and replace. And they're dead reliable.