Schools and even individual classes in a school can skew "smart", probably for some combination of chance and socio-economic circumstances.
My (public) high school class was commented on from grade school as being remarkably bright. But about 3/4 the bright kids were lazy as fuck and got terrible grades (myself included—and I don't mean the try-hard version of terrible grades like "oh no I got one b-", I mean Cs and Ds not being uncommon)—but then killed on standardized tests, 95+ percentile, very certainly not due to application of remarkable effort or even trying all that hard.
My (public) high school class was commented on from grade school as being remarkably bright. But about 3/4 the bright kids were lazy as fuck and got terrible grades (myself included—and I don't mean the try-hard version of terrible grades like "oh no I got one b-", I mean Cs and Ds not being uncommon)—but then killed on standardized tests, 95+ percentile, very certainly not due to application of remarkable effort or even trying all that hard.