Duck and Cover: 1951 Civil Defense Film for Kids
Interesting to consider that this was a reality for school kids in the early days of the Cold War. By the 1970s, when I was in school and aware of such things, such an understanding of nuclear weapons would have seemed extemely naive.
In the mid-1980s, in the field artillery, we were taught to drop to the ground, asses to the blast and hands between our legs. That was for tactical nuclear artillery rounds, but it felt just as silly.
Source and further details: Prelinger Archives, https://archive.org/details/DuckandC1951.