I'm sure some of it is upbringing, but just to add my 2 cents: some of my kids went through JA, and the school does a pretty good job of creating a culture of success in the classroom. I went to public school, and there were definite cliques... you were either an athlete OR a nerd who tried to get good grades. There weren't many who tried to do both. My teammates would laugh at me for answering the teacher in class. I thought it was funny, and made fun of them for being dipshits... But anyway, point is - at JA, and I would assume all the private schools, you are encouraged and challenged by other kids to make good grades no matter what your outside interests are. It's expected, and I think the 'rising tide lifts all ships' theory is definitely at play there.
If you're in that environment, you don't want to be the one dumbass who doesn't get good grades, or score high on the ACT, no matter if you're a bookworm, a cheerleader, or the center on the basketball team. It's quite different from the environment in my public school, which is a good thing. I think over time it does make a difference in average ACT scores on the whole.