Yeah, when I was in high school, we had kids doing that every day for 2 weeks in the last 40 minutes of the day. Everyone got to go home early because by the time they realized it was a false alarm, or shall I say confirmed it, there was only 5 minutes left. It was great. We all would start packing our backs in 7th period because we knew 8th was not going to happen. The kid they caught got busted big time. I think he ended up going back an extra year of H.S.