I'd be willing to bet some (but certainly not all) of them are made by students to cancel school for a day (similar to pulling the fire alarm) - because they know it will get a reaction and bring things to a halt. But I know for sure that these aren't the only cases, and there are some serious psychopaths out there.
In the middle of my senior we had a complete shut down after a guy threatened to shoot up several random schools in the district. It was serious enough to have SWAT teams sent to every school and guard every entrance, fully armed with assault rifles and bullet proof vests. Pics are from my friend:
Last edited: Mar 23, 2016