difficulties of a class. We are mostly talking about the level of depth and skill demanded from the class. In that regard, high school in China is definitely harder than its counterpart in US. Most of the stuff we picked up in high school in US were quite disorganized and superficial. They taught mostly boiler plate way to solve specific problem and the understanding of the subject matter from most kids in those classes would stop at there. You are right that in standard course, most high schooler in China did not pick up calculus until their freshman year in college. But, it is not the same as saying that calculus difficult, particularly high school AP calculus. Single variable calculus is quite a easy subject to pickup. In fact, I did not even attend math AP classes and scored 5 on both AP math I and II. Granted, my experience is about 30 years old, but I do not find today's APs are any different except the naming of it.