I think I'm dumb but I always make good grades?

I've always thought I was the dumbest kid in class, but ever since first grade, I have made straight A's throughout school. I'm in ninth grade. And it's simply not just choosing either A, B, C, or D. I also perform better than my peers on written exams and writing portions of state tests. An example...

Just be more confident in your intelligence. Don't tell yourself that you're dumb.