The thing is that colleges and schools have a criteria for judging every student on a single scale. It is not that one student is dumber and the other is more intelligent. It is just that each one of us carries unique talents and hence, not all of us are good at studies.
However as far as the school is concerned, what matters the most for your grades is your studies. Due to this reason, many students often end up believing that they are dumb though they are not. Hence, such students should focus on honing their other talents rather than on the poor grades.