Some dumb kid doesn't want to go to class and thinks it is funny to bring the school to a hault, so they call in a fake threat. When I was a kid, it'd be a bomb threat. Classes would stop and we'd all evacuate the school to nearby buildings while the cops came in and checked the school. Nowadays, they do the same thing except the threat is a school shooting and everyone shelters in place while the school goes into lock down.
View: https://www.youtube.com/watch?v=wXNbQWdampo
View: https://www.youtube.com/watch?v=wXNbQWdampo