Intelligence is not (only) about knowledge. A person can be "literate" but not "intelligent" at the same time, or intelligent without knowing much. Both is possible. There are people with good grades which I would not consider as clever, they're just fine with being submissive to authority and memorising stuff. Grades are not measures of intelligence, they're measuring certain skills.
Besides stupidity there are many different reasons for being bad at school.
Besides stupidity there are many different reasons for being bad at school.