There is actually a well known phenomena in test scores.  Merchants come up with tests and then they get administered.  At first the scores are relatively bad, and then over the years the kids catch up to the test and then after ten years the tests or re "normed".  This happens with the SAT, ACT, GRE and LSAT as well as tests for public schools.  When they re-norm the test, a grade that would have earned a 750 on the test LAST year might only earn a 575 THIS year... and then the cycle starts all over.  We see the scores rise over ten years and then there is a drop when the new test comes out.

If you look longer term, scores have changed VERY LITTLE in either directions for decades.  You can go back to the 1940's or 1960's and read exactly the same sort of stories about kids not knowing anything.  Kids are just as smart and well educated as every, if not better.  But what has changed is what type of knowledge is valued. Education is now about problem solving and less about facts.