I attended a high school in Toronto from grades 10 to 12. The purely academical content of the science and math courses (math, physics, chemistry, and biology), can seem "laughable" but that is not all that there is to our education system. Every student is required to take elective courses from 3 different categories and students get exposed to lots of different ideas early on.
The public school I went to offered 100+ elective courses on different topics from law, and philosophy to computer science, industrial design, and even hairstyling. We had access to great teachers who took on project-based approaches to teaching that made the lessons more enjoyable. For example, when my Physics teacher wanted to teach us about vectors, he put us into groups and gave each group a sheet of vectors and a tape meter. We had to travel in the direction of each vector on the sheet until we got to a point in the school where he had hidden a sheet of paper and when we took that back to him, he would mark our assignment as completed. After that assignment, everyone in my class had a much better understanding of vector arithmetics.
Students who will attend a post-secondary in other countries will end-up redoing most of the advanced math and science they did in high school in the first-year of university anyway. For the rest of the students, those extra science and math courses will just be a waste of valuable time that they could have spent learning more useful skills.
Having gone through the Canadian education system, I would guess that Canada's high score comes from doing a good job on the lower percentiles, 1-50 or so. The system is slow and repetitive in the lower grades; its focus is getting everyone up to basic standards rather than challenging the best students. But if you're at all clever and diligent, it's boring.
Tracking happens late, at entry to high school (grade 9), where they split you up into academic, regular, and vocational tracks. From there on things move at a more respectable pace, at least in the academic track. But at that point the top-track students are already years behind their peers in other systems that moved faster and tracked sooner. I would guess the top students coming out of high school are about two years behind students in academic tracks in other countries, like the UK or Germany.
The public school I went to offered 100+ elective courses on different topics from law, and philosophy to computer science, industrial design, and even hairstyling. We had access to great teachers who took on project-based approaches to teaching that made the lessons more enjoyable. For example, when my Physics teacher wanted to teach us about vectors, he put us into groups and gave each group a sheet of vectors and a tape meter. We had to travel in the direction of each vector on the sheet until we got to a point in the school where he had hidden a sheet of paper and when we took that back to him, he would mark our assignment as completed. After that assignment, everyone in my class had a much better understanding of vector arithmetics.
Students who will attend a post-secondary in other countries will end-up redoing most of the advanced math and science they did in high school in the first-year of university anyway. For the rest of the students, those extra science and math courses will just be a waste of valuable time that they could have spent learning more useful skills.