And at the same time, higher classes attended highly demanding schools, with very high discipline and demanding curriculums.
Plus, on the 20th century, mass education was adopted by almost all countries (including agriculture based ones), and it had produced people working mostly in office and service sector jobs, not rote factory assembly lines.
So if anything, it's the inverse of the "old wives tale".
(See, we can also agree on some things!).
And? You can teach the average 9 year old to read in 50 contact hours. Quadruple that so they can learn arithmetic (200) and and quadruple that to account for the fact that all school hours can't be contact and you have 800 hours. That's 20 work weeks. Whatever school may be about it's a hell of a lot more than literacy.
I disagree with this common trope. The benefits of mass literacy are extremely obvious; consider that countries where agriculture was and is more important than industry also adopted universal education.