I think in the main it may be because class rooms are over crowded. This is a sad situation, because there must be many students who are just as intelligent and good at subjects as the top achievers. But many kids hit a learning plateau before moving on again into another learning curve. In over crowded classrooms there is often not enough time for the teacher or the pupil to get the best out of each other. I wonder how many kids have left school thinking they were not that smart, when in fact they were intelligent, they just needed a bit more attention and time.
If you can afford a good private tutor to get one to one for yourself or your children it would give huge benefits and probably reduce a lot of stress in the child.