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Question: At what point did high school become a standard prerequisite for university students in the USA?
The United States educational system unlike those in Europe is not centralized and thus the above question varies by state and sometimes varies by county. So while Massachusetts converted the private high school Boston Latin to become the first public High school in 1820, and 7 years later offered free education to children of all ages including free High school as early as 1827; most of the rest of the United States didn't follow suit for 100 years.
Secondly the United States secondary school system was a terminal degree. It was not meant to be a preparatory program for higher Education. So while the United States experienced a High School movement from 1910 to 1940. When communities prioritized High school programs as a way to attract people and businesses; leading to mass opening of high schools. "In 1910 18% of 15- to 18-year-olds were enrolled in a high school; barely 9% of all American 18-year-olds graduated. By 1940, 73% of American youths were enrolled in high school and the median American youth had a high school diploma."
But that doesn't entirely answer your question because most University students did not matriculate from this public high school system, but from private schools. University degrees remained primarily pursuits for the wealthy and public high school programs remained a terminal free degree for everybody else.
What switched everything up was the "The Servicemen's Readjustment Act of 1944" or the GI Bill. This opened the doors of higher education to most of the country. 7.7 million former soldiers took advantage of the GI bill's higher education benefits and 2.2 million of these used their benefits for college. In 1947 Life magazine published a cover story in which they recognized how more than half of all university students were there due to the GI Bill.
This is when public school graduates first outnumbered private education graduates. This is also when Universities adopted standardized pre-requisites to meet and new demand for their services.
So in conclusion, no standardization in US secondary school programs, so the question demands we consider trends. High School Diploma's could not have become standard for College Admission in the United States prior to the High School movement where High Schools graduates became the majority of the population or roughly 1940.. Nor could they become standard for University Admission prior to public High school students being able to afford and becoming the majority in those Universities; and that occurred in 1947.
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