Majority go tutoring but that is not the main reason they do well. the reason they get good marks is not only because of tutoring. These kids are smart, as well as very hard working. From the students I have had from selective schools their work ethic in studying is normally way higher than the average student. To get a good mark being smart doesn’t always cut it. It’s a combination of ability and work ethic
Getting a distinction in AMC without specific study is about the average "smartness". Tutoring is almost like a cult in some schools. Everyone else does it, so you go as well.
As for jazz519's statement on work ethic. That may be true. The difference between selective schools and many normal schools is that selective school students procrastinate then do their work whether or not they're too tired. As such, there are extremely few who sleep 9hrs+ (these comprise of non-procrastinators).
Dr. Du might as well become the head maths teacher for selective schools.