The call-in

http://www.torontosun.com/2014/06/07/teen-makes-bomb-threat-to-get-his-brother-out-of-his-shift-at-burger-king
Actually, this was nor uncommon when and where I went to high school.  It generally occurred on a Friday afternoon, usually in the Spring (nice weather).  The figuring was that once a bomb threat was made, it would take 90-120 minutes to clear the school and check it out before allowing folks back in.  If timed correctly, the call would lead to the early dismissal of classes - by the time the usual safety features could be finished, we'd be past our dismissal time, there was no point in holding us there.  Result - an early beginning to the weekend.