I agree with above - I don't really believe in 'dumb'. There are usually reasons why people don't succeed. Just because you may not have the perfect grade doesn't mean that you aren't smart. It may just be due to lack of motivation, disinterest in the topic or that the way it is taught doesn't fit with your learning style.

I knew of one child at my school who was labeled 'dumb' by some of the teachers because of his grades but when you spoke to him about his interests he was very clued in. One good teacher managed to channel that and make his studies about that interest in ways he could understand. Eg. more visual things, making the math problems about his interests, giving books about his interests with lots of pictures. It took a while, and he still didn't do well with standard tests, but he was able to learn much more and now functions extremely well day to day.