Herd them into a corner, cut the lights out, and lock the door. Not the response I would like to give but the one I am charged to do. By and large, my high-schoolers don’t seem to be traumatized much...
I am responsible for 300 employees as the Emergency Preparedness Officer. We are not permitted to do a drill but during our Annual Mandatory Training I Show a fine video that was produced by the NYS Homeland Security Office. It isn’t perfect but it is excellent nonetheless. I was instructed to say viewing the video was optional but I only had one person leave and he is a manager for a co-located Office. Here is a link to the vid.
https://youtu.be/CVgXH7smBCk