Earlier quoted context omitted.
Because a crazed gunman will (A) go through the scanner and (B) will not open fire when their weapon is detected.
Gun and weapon detection is still useful to stop shootings and stabbings that arise out of fights, where one member may suddenly present and use a previously concealed weapon. Agreed that the usefulness against a crazed gunman is less clear, although crazies do not always have a rational plan. If the weapon was concealed in a backpack or something the security may be able to tackle them before they access it. If they…
Which might be a reasonable thing to aim for? It was mentioned up thread there's >100 school shootings/yr. The vast, vast majority (>>90%) of those are not the "crazy person randomly shoots up school" type, but rather the "two people in a fight / two groups in a fight / targeted revenge on single person".
The former is tracked as an active shooting incident, by the FBI. In 2023 there were 3 such incidents: https://www.fbi.gov/file-repository/2023-active-shooter-repo... (10 year average is a bit higher).
Of course the downside with the latter is that simply weapons checking at the school won't stop it from happening right before / after school, or otherwise outside of school.