4:12 is not impressive at all for a 19 year old on any kind of mileage. That's what grown men are able to do.
The real issue is his age. He's a 19 year old running against 17 year olds. This is a huge advantage.
Now I don't have a problem with 19yo being in high school. After all, the mission of public education is to educate (at least attempt) as many young people as they can - including developmentally slow students (not saying he is). With that said, I'd say it's fair to allow 19yo to participate in high school athletics. The physical, social and emotional development advantages are obvious. But I'm not for counting the performances of of 19 year olds as high school records. The unfairness of it is obvious. He's a grown man among boys.
FWIW, when I was nineteen I ran 8:40 in the steeplechase, which is far better that the hs national record. That would have been completely unfair had I been racing against hs kids -- even though as a nineteen year old, I didn't look a day over fifteen.