Older Florida schools had a very open campus style: just covered sidewalks and one door into a classroom. Even newer designs aren't anywhere near as secure as what you're talking about. Going to the office to sign in is more or less voluntary. I doubt there was any way to know this guy was even on campus.
I reported a kid last year for posting online threats. He was arrested and taken for involuntary psych eval. He faced the possibility of criminal charges which were not filed, and was expelled. There are two choices: online home school program, or attend a school with all the other problem kids. Since he was in middle school I doubt he's expelled forever.