I took the CA proficiency test to graduate at 16 and went to college. It was the equiv of a HS diploma and after 1 semester at college I never needed to show/prove I had a diploma other than to get into my transfer school. After that, the fact I had a college degree trumped the need to prove I had a HS diploma or equiv.
With a plan like this I could have been in college at 12.
One of the problems with not having a high school diploma was they were not allowed to apply for scholarships. We did not need loans but without a high school diploma that was not permitted, either.
Also..some homeschoolers have told me that the military have high school diploma requirements as well.