So much security theater just to allow someone like this to waltz into a classroom to openly berate children.
I don't see the "so much" security theater you're referring to, and it seems like she got very lucky. She appeared young enough to blend in with the other students but also appeared old enough to be a substitute teacher and had attended the high school not that long ago (and conveniently lived right across the street).
During her scouting the week prior, she "heard other staff in the main office talking about the substitute teacher agency and told the secretary she was a substitute", so she had that established ahead of time. On the day she went into the classroom, eight of the approximately 70 teachers were absent, and one of those positions happened to be unfilled by a sub (something she couldn't possibly have foreseen).
Incidentally, unrelated to your comment, here's the local news source instead of the trash tabloid Metro.