Is it high achieving kids or a high achieving school? It's probably both. Since SAT scores (and I would assume PSATs) correlate to some extent with socioeconomic status, I would guess that the demographics of the school plays a role. The kids are ones taking the test, so obviously they get the credit for high scores, but I think there is an interaction between the two.
As for which school to choose - that's hard to know. I guess I would choose a school where the curriculum prepares them for college, but not where the kids compete against each other in a vicious way. That sounds too stressful for even really driven kids.
