If you look at hockey rosters, you’ll find a lot of 20 year old freshmen on the teams, either because they’ve been playing juniors, to a PG year at a prep school, or just were older than the average 18 year old hs grad. One of the guys I went to hs with who played hockey turned 16 in Sept of our freshman year (and got his driver’s license!). His Sept birthday was after the cut off for K, and he (like many boys in our school system) repeated 3rd grade.
Joe Pavelski (Sharks and Stars) did play at this same high school (his dad played on the same teams as my classmates) and then directly to U of Wisconsin, and then to NHL, so it can be done.