The problem here is that even though these kinds of tests have been going on for decades they weren't published online. So older results may need to be looked up in reference books. Also these types of ratings are different for every subject, like math, science, literacy etc. I am not sure if there is a generalized score provided by everyone to test overall education level.
We are 14th in the world, behind countries like China, Germany, North Korea, and I forget the rest. The new thing now is lawmakers are planning on eliminating summer vacation in favor of year round schools, there are about 2000 such schools already!
I think it started happening in the 1950s/60s. Students grades started dropping dramatically in this period. So, to try to cover this up, they dumbed down the test so the students would score higher. We still aren't as good as we used to be, and it doesn't seem like it will get better anytime soon.