Turns out the answer is interesting - about 50 years ago, the graduation rate for high school was around 50%. These days it's much higher (90% or more in some areas).
Does this mean the children is learning? Perhaps.
It also may represent a failure of our systems - in the past, not everyone needed a high school degree to be a functional member of society, and forcing those people to "graduate" may in fact be a net loss to society.
In other countries, this is explicit: people are tracked to "educational" vs. "vocational" career tracks earlier, and there is not the emphasis on graduation rates as a metric.
Kids also drink less alcohol, commit less crime, get teenage pregnant less often. So yeah, they are likely to be learning more then in 1970.