obviously none of you went to a school with "arches"
I went to an American inner-city public high school with gang problems (after something called "forced busing" was enforced). I left the inner-city soon after graduation (1998). Yes, the gang members could have thrown their guns, knives, etc. over the wall and through the fence, etc. After the metal detectors were installed, though, the police officers at the school did not have to deal with nearly as much hallway violence. It was noticeably safer for me to be in school, and the detectors never went off on me despite my school gear. Yes, things still happened, but most of the goons obviously kept their violence away from the school at that point.
Anyone who wants to attest to why this happened is, of course, speaking off of opinion and not fact. Of course, that does not seem to stop people from doing so. Perhaps the trouble makers decided there were better places to fight and oppress? Maybe the real effect of the detectors was deterrence, which is based more on psychology than "stopping power".
I suspect the British authorities looked at the "success" of American school metal detectors for the idea, or for an example. As an American, I am embarrassed that measures such as metal detectors must be used. Perhaps people should stop being angry about how effective the metal detectors could or could not be, and start being embarrassed at how bad things have gotten. The real problem in both countries, after all, is not the detector.