I'm confused what make top 50% possible in this context mean. Pls help.

As a teen, I struggled in school. Let's say I was one of those students who made the top 50% possible. Because I was a below-average student, I thought that I would be below average at everything else I did.

I know that this guy didn't get good grade, however what "make the top 50% possible" here mean? 50% of what? Pls explain.

Thank you