When entering the school, students' backgrounds are very different. Some people come from humble schoolhouses with flocks of cattle and sheep in mountain villages, some people come from first-class middle schools in big cities (even the history of middle schools is longer than ours), and some people compete for gold medals. The difference in background directly leads to the gap in knowledge. In our computer department, some students have never seen a computer before entering school, and some have become programming masters after years of training in informatics olympiad.
What impressed me more was that I learned C language programming in my freshman year. On the day of class, the teacher invited several students of the same grade to tell everyone that they were admitted to the school by competition. This course not only didn't get full marks, but also was responsible for testing other students. What is even more exaggerated is that although these students have reduced the difficulty when they set the questions, for most other students who have no foundation, the questions are still too difficult, and everyone's grades are in a mess. This is my earliest memory of crushing feeling.
After many courses, students who already have the foundation of the competition (our department is a master in the cloud, and there are gold medal winners in other disciplines besides informatics) can naturally cope with it easily, while students in areas with scarce educational resources are still trying their best to achieve nothing. I don't believe this situation is caused by IQ.
In addition to background differences, everyone's efforts are also very different. In my impression, academic performance is positively related to the degree of effort. I have never seen a classmate make extraordinary efforts, but the result is poor, and I have never seen a classmate easily lead without hard work. On the contrary, I have met some students who have a halo flash, but for various reasons (such as indulging in games), they missed too many classes, which led to a sharp decline in their grades.
Although this is a memory close to ten years ago, I believe that the situation today will not be fundamentally different. In my opinion, none of the anecdotes described in the top post can show that IQ is the key factor. As far as the average academic qualifications of students in our school are concerned, I believe that as long as there are enough educational resources accumulated and enough time invested, it is not difficult for everyone to get good grades. It's just that fate is unpredictable, and neither can be forced.