After the Parkland Shooting here in Florida my suggestion was to have Metal Detectors installed (in schools)
Metal detectors aren't without their place, but in the case of Parkland, there was a top-down failure to act on a nonexistent plan in the first place. I think this holds true for a lot of the higher profile school shootings as well. Columbine began outdoors, beyond the reach of metal detectors, and in Lanza's case, he shot his way into a locked school. Getting back to Brooklyn, it's worth noting the preceding incident that prompted the screening which produced the two dozen weapons, and the fact that it was office staff, not a metal detector, who spotted the gun in the kid's backpack. Not that either is right, but there's also a difference between a student who shows up to school with a gun in their backpack and one who shows up to school with a gun
intending to indiscriminately shoot people with it.
As someone who works in schools, it sounds much easier than it is to competently screen students as they come onto campus. What this really boils down to is a staffing issue, and schools were already notoriously understaffed prior to the pandemic and it's even worse now. Not to deviate too far from the intent of THR, lack of staffing, particularly counselors and support personnel, are at the core of a lot of these issues. Literal security, not so much.
I'm pretty sure the OP's intent here was to present this as a failure of laws aimed to prevent minors from accessing firearms, particularly where those laws tend to be very strict. But even that isn't looking at the bigger picture. Besides, the counter argument to that is in the absence of those laws, there would simply be
more incidents like the Brooklyn kid with the gun. Citing one failure of the law isn't a compelling argument that the laws aren't working --again, just playing devil's advocate here.