
That sounds like a nice discovery. You want to believe that with a lot of hard work, anybody can rise to the top. It is not fair that some people are smarter than others. This is not something you can control. But you can have grit and will yourself into a better student.
Here is what strong self discipline will improve: standard test scores, quality of schools accepted into, number of school absences, amount of time spent on homework, and amount of time spent watching TV. Not only will self discipline improve these things, it will itself gauge how well you will improve in each of these areas.
Not many of the tired old excuses apply to success in school. Don't blame large class sizes. Don't blame books that are not interesting. And don't blame the teacher. You have the ability to improve your self discipline with some hard work. It will pay off in the end. Academic research backs this up.