If ever a reliable method for measuring actual intelligence rather than IQ is invented I imagine we'll be seeing a somewhat lumpier graph than that smooth mean distribution curve.
It's amazing you used standardized test stats, while I believe the test are part of the problem. When I was in school, you learned the subject, and the standardized test was a decent level. Now, all the subjects are should be called reading comprehension, because that's how they teach. Teachers are held to teach their students how to pass the test. Extra school funds are tied to percentages based on test scores. So they pass out, and teach off of, worksheets that are mirrored off of these test. So they don't teach science, hey teach you to answer the multiple choice questions after reading about science. Everytime my kids bring homework home i ask them if all of their work is like this, this being reading comprehension worksheets, and they say "pretty much".