I did a similar experiment in 1982, except it was with an final exam in Algebra II I gave in 1967 to a 1982 calculus class as a first exam. Every kid in the calculus class had an "A" average in his/her math classes. of the 16 kids in the calculus class, two got "A"'s; none of the others got above a "C" and about half failed.
One only has to look at high school textbooks from the 1960s and compare them with those with today's book to see that much of the content has been removed. But I must say this has been a continuous thing. A second year algebra book published in the 1930s had much higher content that one of the 1950s.
The math textbooks may be the worst aspect of the schools. One Freeper told me it's as if they're confusing on purpose. And it did seem that way to me.