In 2019, and I’m using this because you can explore it at https://www.nationsreportcard.gov/mathematics/states/groups?... , Mississippi’s 8th grade math mean score was the same as New Hampshire’s, when comparing white students. Mississippi is just way less white than New Hampshire. California, also near the bottom overall, has better white performance than New Hampshire, but not as good as Massachusetts, which has always had a lot of smart people moving to Boston.

The ranking there is simply a diversity/brain drain ranking. Mississippi and California are way more diverse, so they rank low. States with nothing to attract or keep smart parents, like West Virginia, rank low too.