Makes me wonder what type of "training" the teachers had received, given there is no mention of harm befalling the attackers. I’m particularly surprised the school’s insurance company would allow them to perform this stunt, considering someone could easily been hurt.
I often work late at the office. I have warned my coworkers to always announce their presence when they enter the building through the front door, which is out of view, and tried to impress on them the announcement is for their safety more than mine.
A lack of planning on your part does not necessarily constitute an emergency on my part.