Yeah, the fact that they were all looking into the camera is odd and the first sign that something is off. Highly doubt a teacher would be that bold in the middle of a classroom with everybody there.
Second is that everyone is already standing around.
I'm gonna guess that they were being shown first aid (cpr, heimlich maneuver) and some of the kids were acting a fool so that somebody could be caught in a bad position. High school kids would especially be prone to goofy behavior during these demonstrations. You can practice CPR on a mannequin, it's a little harder for the Heimlich maneuver.
Disclaimer: This is not a post condoning teachers having inappriopriate relationships, this is a post trying to find the most positive scenario.