Many school systems in the U.S. have a 9/1 cutoff date. So your example oversimplifies "True" graduation year dates.
Statistically, we should see up to 1/3 of players in a graduation year with a date the year earlier than you're putting forth. E.g., a third of players in 2020 would be born in 2001 (born in the 4 months between September 1, 2001-December 31, 2001), and two-thirds in 2002 (born in the 8 months between Jan 1, 2002-August 31, 2002.)
Of course, on teams like the 2020 Crabs, you see more like 80% born in 2001. That's called gaming the system.
