Demographic change is the obvious explanation.

Score changes look about the same across performance percentiles and ethnicities. That suggests it's a systemic issue unrelated to population makeup. While I'd be interested to see regional and economic breakdowns, it's certainly far from obvious it's a result of demographic change, especially after such a short period of time.

Mississippi is one of the recent education success stories though.

I wouldn’t be so sure. Anecdotally all the kids these days seem equally messed up. It could be that the Chinese and Indian kids are propping up the locals.