Massachusetts has the "smartest students" while Mississippi has the, uh... least smart, if you believe National Assessment of Education Progress test scores are indicative of intelligence. One unsurprising trend: States with lower levels of child poverty have better test scores.
I taught in Massachusetts for a while. The tests are great for under-performing kids and terrible for the over-performers (or whatever the opposite of under-performing is), because the need to excel on the test and the funding, etc., associated with the results leads to a "drop everything and teach to the test" even for kids who don't need that, causing, in my opinion, a regression to the mean.
Maybe mediocrity for all is the goal, since that means a lot of kids are doing better than expected, but to me it seems a broken model. The kids who need the help should be able to be helped up without tugging other kids down.