The only time ever a school I went to was locked down was 8th grade. Some guy spray painted the bathroom and nothing more. School officials decided it was time for a lock down (no bathroom, walking around, etc.). They planned to bring in all students starting with the most suspicious and the biggest troublemakers.
The guy who spray painted the bathroom was in one of my classes, and I heard he cried when they found him. I almost got into a fight with him one day, but that's irrelevant.