When books were the only way to find out about the world, or the only way to escape to another place or time for an hour, well then you damn well better learn to read. And they did.
It's documented in the test score comparison between WWI and WWII recruits how badly literacy had fallen in a few short decades. Look it up.
Teaching kids to read isn't hard. Marva Collins ran in an inner city school, and said that she never had a pupil that she could not teach to read by Christmas of the pupil's first year. But she used phonics, which have been around and working since the days of Martin Luther. Just because today's standards are very poor now doesn't mean that they weren't better before.
There has been a lot of progress in many areas since Martin Luther. But none in education in the last 50 years. If you've read Brave New World, you'll recognize the philosophy of the progressive educators. They believe in different training regimes for different classes, the Alphas, Betas, Deltas and Gammas of the brave new Progressive era. Read John Taylor Gatto. Not only are today's standards much lower, it was a planned thing.
Charlotte Iserbyt's The Secret History of Western Education is on the same subject. If you want it from the Fabian horse's mouth, research the writings of John Dewey.
There is a direct correlation between years of schooling and overall knowledge. I doubt the recruits of WWI were better educated than the WWII recruits who had more years of schooling.