Unless the Germans have changed drastically since I was last there (long ago), their system would not be tolerated in any US school system.
What Germans and most other countries do is very early on, at young ages, identify which children have serious academic promise.
Those get special educations in different schools, the best their countries can offer.
Everybody else gets the average-kids education and neither competes with nor is tested against international standards.
Americans are the only country that tries to keep all its public education kids together in one school and in the same classes.
The result is, on international comparisons, our average kids measure poorly against other countries' advanced-education kids.
But even within US public school systems there are huge differences in test scores from one to another, depending on what kinds of kids go there.
Bottom line: alleged changes in overall IQ are more likely to represent differences in exactly who is being tested and how.
Our modern education system was imported from Germany in the late 1800s, part of the “progressive” movement that influenced people such as Teddy Roosevelt and Woodrow Wilson.