As other elementary schools across the country were preparing for the new school year by cleaning classrooms and training teachers, Hermosa Elementary, in Artesia, New Mexico was also installing a network of wireless microphones that could pick up the specific concussive audio signature of gunfire. Placed high in classrooms and hallways, the golf-ball-sized devices can alert authorities to the sound and location of gunshots, reportedly within 20 seconds of firing. They can also identify make and model of guns, and automatically lock doors and sound alarms throughout the campus. They are a technological balm for a terrifying problem: In the wake of the Parkland shooting, and Sandy Hook before that, school districts across the nation are spending hundreds of thousands to outfit campuses with high-tech surveillance, crisis response, and police technologies. Playgrounds are cordoned off by biometric locks requiring face and iris scans, parking lots are scanned and license plates are recorded, gunshot-detection devices are embedded in cafeterias, human police wear body cameras, and autonomous robots patrol hallways to detect weapons.