Someone called one in when I was in school, around 1983-84. They evacuated, and had the bomb squad come by to check out the building, with the fire department standing by-- it was taken very, very seriously. The rumor going around the student body was that there was no bomb threat, and it was an excuse to search our lockers.
Fortunately, it was a warm day, because we were outside for about 2 & 1/2 hours. People were napping on the lawn, and lighting up cigarettes, and teachers weren’t saying anything. It totally messed up the lunch periods, and I think we ended up missing some more class as lunch ran into an afternoon class. IIRC, we went to our first period class, then our afternoon classes were redone so we went to 45-minute versions of them.
There were so many different rumors over the next week regarding who phoned it in, that even though I recall them catching the person who did it, I don’t recall who it was, nor what the whole story was.
I do remember that at the time, we all thought it was weird they were taking it so seriously. Any of us could imagine calling a threat in for a joke, so we were all sure that’s what had happened.
And it did turn out not to be a real threat, but it wasn’t considered a joke. I’m pretty sure the person who did it got into some serious trouble.
I sort of wonder if taking it seriously like that was intended to discourage copycats. I know some people were sure that at any time, we’d just get dismissed for the day, but nope, they kept us outside, on the lawn, with nothing to do all that time. And when A lunch rolled around, they wouldn’t even let people with that lunch leave, since no one could prove they had that lunch, so no one could walk over to McDonald’s or wherever. It was really kind of an ordeal.