I did well in my AP calculus, 98/100 or something like that, but I still felt the pressure in all the tests and exams, as there were just so many questions crammed in a single test. I had to keep writing, non stop, and barely had time to double check any of my answers. Contrast to my school experience before I went to Canada, I could usually use way less than half of the exam time to finish and double check every problem.
The same pattern continued in U of T. The class average of courses like algorithms often went below 40/100. I usually had to keep writing until the last minute to finish every single question in an exam. Assignments were usually time consuming and required lots of thinking. I still remember the tremendous satisfaction when I finally proved a theorem about universal hashing (an exercise from CLRS, IIRC). And past success on assignments and tests didn't guarantee good grades in the final exam. I was actually dumbfounded when I did poorly in my AI course, even though I did extremely well except for my final exam.
Looking back though, I am very grateful for the struggles my professors put me through. They made me. What I learned then carried me a long way, even today. Without the high standards imposed by the schools, an ordinary student like me would not be able to do as well. As I kept telling friends, schools MUST keep their students in discomfort zones to help them learn and improve - the very thing that US schools, in particular SF bay area public schools, have been failing miserably.