We had a lock-down at my High School once. We were in the Catholic system, and the public High School across the way had some kind of incident where drug paraphernalia (almost definitely a bong) was found in a kid's locker. That was apparently enough to have us stay locked in our classrooms for an extra two hours before we were dismissed.

Same sort of thing happened again later in the year, but after school hours, when we were supposed to be hosting a film festival. There was an assault or something at the public school, so I ended up locked in the chapel for a few hours with all of the other people who were volunteering in the festival. At one point the police escorted us to the teacher's lounge so we could binge on the snacks they had locked away in there.