There are, in place, municipal, police, commercial systems to pinpoint shots fired, pretty proprietary, but if the kids have a different method, more power to 'em! They do have some serious motivations!
Ultimately, they came up with SIREN, a 3-by-6-inch device that screws onto ceilings like a smoke detector. It’s equipped with a microphone and a computer program trained to identify a gunshot
I’m sure there’s more to it but just about everyone on the planet is carrying around a microphone in their pocket. In that same pocket is a GPS device which can be accurate to within 15 feet, and could be even more so - down to 30 inches - with a bit of additional processing power, allowing “the device” to know if you are inside a building or not.
“Training” a microphone to detect the sound of a gunshot is trivial, and creating a system to falsify reports would not be that difficult either. (Multiple reports from nearby phones? Alert the police. Single report? Likely a false positive.)
So I wish them luck, but I’d say “an app” is likely to be easier, vastly cheaper to deploy, and more capable of “reporting” to authorities in an instant. Of course there would be privacy implications of having a microphone “always on” in some respect for detection purposes, but then this “smoke detector size device” is always listening too, so…