They invested in a host of security measures. Lock-down infrastructure, surveillance systems, metal detectors, body scanners, improved lighting, perimeter fencing, resource officers, even arming teachers.
Such measures were effective. But they were also expensive, and many school programs and activities had to be sacrificed, from bus service to sports.
And for most kids, going to school became an ordeal, an ominous, joyless, daunting experience. Schools became hardened. But so did a generation of young people.