I had an exchange year during High School where I moved from Germany to Florida for a year and attended a public school. I hate to say this but to me the classes were laughably easy. While I was a 10th grader in Germany I could have easily graduated there.

I do however favour the US system of individual classes where you could advance in e.g. Mathematics but if you had trouble with language you could take a less-intensive course.

This doesn't work in Germany. You either advance with all subjects at once or you don't. One of the most stupid things that I have witnessed. In our 11th/12th grade we would have a huge difference between grades. About 20% of people were getting straight A's while over a third struggled to even pass. Barely anyone in the mid-range. Terrible system which makes it really really hard for teachers to teach a class.