This is not really true is it? There were not that many standardized testing globally to measure such claims. Many people were in poverty and did not get tested, did not go to schools, or finished schools very early (5, 9 years). Many more kids go to school these days.
> In the Soviet Union more time was spent teaching mathematics and a whole culture developed around mathematics being fun
It is just wrong. It was the same as now, except it was critical for people to show results because otherwise you had grim perspectives in the life, there was little "fun". People wanted to get into universities to get better jobs and to get better apartments, to be able to leave their parents. You could not just buy places, but a good position in some public body would guarantee you a nice place. FYI engineers could earn more in comparison to other jobs, not to mention if you could get into defense industry.