Every Friday afternoon.
Right after lunch, from '56 ,first grade, til '62. Buzzer would go off, and under the desks we went.
Of course by the end (62) we all said pretty much the same thing.
"Oh yeah, bend over, put yer head buhtween yore laigs ........
and kiss yore ass goooooooooodbye."
The drills were quietly discontinued fall of 62, no explanation, they subbed a fire drill for it.