Despite prompt law enforcement responses, most shooting incidents were stopped by means other than law enforcement. (Suicide or “civilian” intervention.)
Some commentators ask about metal detectors in schools. Opinion seems divided. Aside from cost, the technology requires trained operators, and should be positioned at every entrance with one or more enforcers to deal with contraband. But while detectors may reveal attempts to sneak knives and guns into school, a dedicated assailant will blow past the device or possibly shoot or stab the operator on the way inside.
(Knives in schools are seldom addressed but in Kunming, China, in 2014 eight Islamists killed 31 people using only blades. Nearly 150 victims survived injuries.)
An increasingly common discussion involves a school lockdown with students kept behind locked doors. But a clever assailant would wait until the interval between classes or, perhaps “better” yet, when school lets out. Even if most students make it to a safe area, others inevitably will be caught in hallways or in the open with nowhere to go.
Arson and explosives remain the deadliest agents of school killings. In 1927 the Bath School bombing in Michigan killed 38 children and six adults—a greater toll than any school shooting.
(In a worse example of arson murder, in 1990 a jilted lover killed 87 people at a New York social club. His weapons were a plastic bucket with $1 worth of gasoline, and a match.)