The 1950s were the years that the rate of decline in the ed system began to accelerate, when that b&st&rd Dewey's idiotic notions began to be accepted wholesale. The 1960s brought the start of increasing unionisation, and it's been straight down ever since, for the most part.
If correct, it would reflect the fact that the decline began when the average American of the time was educated, during the 1800s. In fact, that is when modern government school was invented, although, as you say, the baleful philosophy underlying government school didn't dominate thoroughly, even in the government schools, until beginning in the 1960s or so.