My great grandmother graduated in the 9th grade. Her teacher decided that she and her younger sister were ready, gave them a test on the blackboard, and sent them home with some sort of graduation certificate.
This happened in the middle of the school year back in 1912 or so.
My grandfather on my dad’s side was pulled out of school after the 8th grade to work on the family farm. Afterward he successfully ran several businesses over the years, including the largest John Deere dealership in the Upper Plains.