Personally, it has never happened at my school.
However, it does happen to one of the local high schools (I don't go there, and probably never will), at least once every few months. They used to be taken very, very seriously. The students couldn't go use the bathroom without an escort, which meant the VP's were running like hell all over the school. Lately though, because it's easier to trace the calls, which are usually made by students, the building is not evacuated. They just search while the students work.