Perhaps in early childhood, doing well is a decent indicator of intelligence. As young kids, good grades give an idea of how fast a child may grasp simple concepts like basic arithmetics or reading and writing.

Later on, (like High School and beyond) good grades just give an idea of how dedicated one is. It has more to do with other factors such as amount of time spent studying, discipline, access to resources, etc.

If 2 kids have similar aptitude, the one who studies more will get a higher grade, this does not mean the one who studies more is "smarter" it means s/he has more KNOWLEDGE because they dedicated more time toward the subject.

Intelligence should be measured in the ability to problem solve neutral topics and the ability to reason, not how well one can regurgitate facts on a standardized form.