Are you aware of the origin of 9th-12th grade education? John Dewey was probably the most aggressive promoter of it; widespread 9th-12th grade education didn't exist before the 1920s (people just went straight to colleges/universities who wanted to keep pursuing education). Groups that resist change, like Amish or some Mennonites, ended up sticking with 8th or 6th grade education.
High school education really took off in the 1930s since it was proposed as a way to reduce unemployment without all those youth out trying to find jobs and provide teaching jobs. The model of secondary education in New York City was viewed as a success in doing the same with large amounts of immigrant youth in the 1920s. We basically embarked on a national experiment to the point that now everyone thinks that sitting in a classroom until you're 19 is normal.