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.
That said, yes, there was a dramatic increase in attendance and graduation from ~1900, when the graduation rate was about 6%.
US Department of Education, 120 Years of American Education: A Statistical Portrait, p 55, "Table 18 --- High school graduates, by sex and control of institution: 1869--70 to 1991--92"
<https://books.googleusercontent.com/books/content?req=AKW5Qa...> (PDF)