Where in the US do we have a problem with random people hanging out in schools undetected? Pretty sure that human face detection would spot that pretty quickly.
Not to mention the strict ID badge policies at most (if not all) K-12 schools. All faculty and staff wear ID badges, and visitors are required to sign in and get temporary ones. An unfamiliar adult hanging around without one will get questioned very quickly at most schools.
I don't know where you live, but as someone who frequently visits schools, I find ID-checking superbly lax.
While I don’t think the referenced technology is a solution to the problem, think a bit about the first question that you’re asking.