SAT scores and other measures of intelligence are not affected much by the variation of quality in schools within the U.S. or by other variables that parents can control, so I don't think your children will be harmed by attending a high school with few National Merit Scholars.
As a former PG kid from a poor family who went to a rural public high school, I have some experience of this. I was busy educating myself in high school, reading Voltaire in biology class, and so forth. For me, the real problem with that setup was the lack of a intellectual peer group. I arrived at college emotionally and socially immature and with abysmal work habits.
My psychologist buddy tells me that most people have comparable SAT and GRE scores, but my GRE verbal and math were each exactly 100 points higher than my 98th percentile SATs, after a liberal arts education (GPA of about 2.5) and a few years of work experience.
