Depends on what kind of grades. I am serious, one of the kids in my grade is flunking appropriate-level Chemistry and Biology but has a Gold in UKChO, BBO and participates in HOSA. The reason? AP Chem and Bio requires a tremendous amount of English comprehension and writing skills and that guy is not good at writing. In other words, he gets the point but is unable to convey them on a test sheet of paper that earns him points in a consistent fashion.
Another kid participated in AIME and is learning university level mathematics but not using English textbooks, because again he is not good at English as he is at Mathematics. For this reason he sometimes lose big points in some subjects not because he miscalculated the value of something, but merely that he didn't know what the value is for because he didn't understand the question in the assigned time. The lack of some skills really truncate the latent perspective for others and we can't just say one is equally stupid as another when one kid has 2/3 required set of abilities and is just missing the last step to glory and the other one is not trying.
Good that my times in DArt has served me well when it comes to writing. At least I don't have to be labeled "not smart" when I am trying to be smart.