Not every urban school has metal detectors. In fact, a great many such schools can nowhere near afford such things.
They do safety the old fashioned way. Everybody is packing.
Most kids in urban schools have no interest in "packing." Many have enough painful personal experience to know shit like that is not a game.
It isn't a "game" in the inner city, its a matter of survival and always has been.
I went to a good private, Christian school , but many of the guys in the neighborhood went to the government school system in the major city i grew up in.
I saw one of my age peers just a few years ago, and he was so proud to still have the Switchblade his dad gave him for his first day in high school. I heard all the stories about boys getting shivved in the boys room. At my school , we did do pranks , but the only real violence was from the priests and brothers who ran the place. At government school, a wrong step, you could get literally killed to death. An Italian I know lost an eye ball in the government high school lunch line for allegedly "cutting in".
The best thing I can say is that the horrors of the Government School system provide a deterrent for private school children to stay in line else they get expelled and sent to the living hell of government school