According to this ...
... roughly 75% completed high school in 1960. Yes, the rate is higher now, but to say "the vast majority never came close" in 1960 is not true. Is Griswold trying to imply high school in 1960 catered to group as exclusive as today's Philips Exeter?
What might be a more accurate and responsible statement is that high school diplomas did not require three years of math; many students graduated with two years of math. Which meant that there were many students who took only algebra 1 and geometry, but not algebra 2 and precalculus.