How can I get enough sleep while keeping up great grades, having a social life, and having spare time to do other things?

— Forever tired
As the internet meme goes, high school students can only attain two of the following key elements of life: social life, grades, and sleep.
However, we are complex individuals and desire for more; we want all three.
Unfortunately, a fact of life is that achieving balance is incredibly difficult. Thus, the solution is to pick two, while realizing that the two selected options are quite interchangeable on a daily basis.
For instance, say it’s summatives week, and you haven’t even started your project yet. You may want to pick “grades” and “sleep,” and put off the other option. You can then finish your summatives, while simultaneously getting enough sleep to ace exam week. Then all your schooling will be over, and you will have all the free time in the world to do whatever you want with your friends. Thus, you’ll have succeeded in all three categories: you’ll have excelled your exams and summatives, gotten enough sleep, and finally kept up your social life, as well!
So all in all, the answer is to have a strategy: know which of the options (social life, grades, and sleep) are most important at the moment, and sort out your priorities. This will allow you to lead as balanced a life as possible.