NBC-CT interviewed one student earlier. The female student said she heard 7 ‘booms’ before the gym teacher had students huddle in the corner. Student said she heard additional ‘booms’ but didn’t not say how many, while they were huddling.
Huddling in a corner doesn’t sound like a very good idea if you can’t get the kids out altogether. A pile of kids in a corner is one easy target but a classful running around in the gym like they were playing the most frantic game of dodgeball ever would be much more difficult.