jwbot was right on. Younger JW kids are more advanced in school, generally. Being forced to study, sit through meetings and read names like Maher-shalel-hash-baz will do that.
But there is a DEFINITE leveling off after middle school and most certainly after HS.
Bradley
PS -- Interestingly, I heard the same thing about other "fundy" kids on a different forum (non-JW, mind you). The same thing already stated could be said for 7th Day Adventist kids, Mormons etc.