Students are over-studying in subjects they are good at. The aim is to increase your ranking in a subject within your school.
You want to be as close to the top of the class as you are able. If you are already in the top 5 or 10 students for that subject, you can then be strategic about how you allocate your time.
If you have a lower ranking in a particular subject. this is where you should focus your effort. For example, if you are getting 92%, it will take a lot of effort to get to 95%. You would be better to take another subject where you are getting 70% and put in the effort to get over 80%.