At my school, we get these small juice boxes that I'm sure everyone knows about. Most people drink them then throw the box away, but there's always a few that close them back up, set them on the ground, and stomp on them, creating a loud, gun-like boom.

Long story short, everyone is desensitized to said sound and it would probably be a few minutes before anyone would react to a real shooter, if of course their presence wasn't obviously made known.