This process took decades. By the mid-1960s, still only about half of states offered funding for kindergarten programs. The final growth spurt took place over the next decade, according to a study by Dartmouth economist Elizabeth Cascio. Many states especially in the South and West saw kindergarten as a way to ultimately save money.
“[States thought] this is going to reduce grade repetition where we have to pay for a kid to go through this grade twice,” Cascio said.
By the latter half of the 20th century, the K-12 school system had been largely established.
This melding of kindergarten with the broader education system led to a kind of natural merger. By virtue of being in the same buildings and under the same administrative auspices, kindergartens naturally became more academic.