I was one of those kids with the very high SAT scores (both parts) and ACT scores (all parts) early on, and my scores didn't change much from the end of middle school to the end of high school when I took the tests again.
I think that the ceilings for these exams are pretty low for students taking them at the end of high school (sort of like grade level tests in grade school). Also, as was pointed out, the SAT bases a good portion of the score on reading comprehension, which isn't used much for verbal reasoning and IQ scores.
