I guess you had the chance to enjoy one of our wonderfull religious classes then^. I completly agree they suck. At least most of the time they do, most people select ethics instead of religion once they have the chance to do so. There is also quite a debate about those classes in general because it doesn't really fit in with the seperation of church and state (however that's a problem in Germany in general...) I had a good experience for a few years when I had an awesome teacher, he was also politically active and in my city's parliament for the social democrats and his classes got quite some people interested in local politics, at least temporally. In general his classes were always interesting.

I completly agree whit your experience with our math classes, I was always a rather good student in math, never the best but one of the best 3 or four in my class and suddenly when I enterd the second last grade all marth students who were better then me went into Math LK (ap classes more or less) and I remained alone in a math class which was laughable really. We do the same stuff for weeks, just straight up repetition. I still do my homework and show up to classes but it really feels like I could just stop doing homework and still be good. This makes me especially sad because there are other classes (history for example) were we are always working on a tight shedule and there are still so many subjectes we don't learn anything about that would be vital. (By the way this is also the single subject that most of the students I met in America are the worst at. There were multiple people who asked me about the Nazis or Hitler without realising basic details, Hitlers death for example (one of the best moments was when a friend of mine was asked how influential Hitler still is in Germany politics because he is never mentioned in the news anymore, he asked if JFK is still doing well or if he still has these problems with headaches))

Most of the things you say sound correct, althought the PISA test is also used below class 10, and that is where the people who don't want to go to college leave school so that is only partially responsible. In addition to that some of the questions on the PISA test that I had to answer were also clearly not related to the things we learned in School, especially the question concerning literacy. I was the person who scored best in my class, especially in literacy eventhough I had normal grades in the usual German class. Most of the stuff that was asked was something that I learned at home, both my father, stepfather and my mother all read a lot and always got me books/took me to the library etc and generally talked to me a lot in German, the children who had immigrant parents probably had way worse scores for no other reason then that their parents don't speak German well. But ofcourse there are probably more immigrants who speak another language at home in the US then there are in Germany.

Also I'd be careful to draw a conclusion between the top-of-the-world higher education that the US certainly posses and US high schools.