Sorry to hear about your grandsons bad marks in school! He may be the best student academically, but if his behaviors are disruptive and perhaps they detract him from completing his work on time or at all, his grades and academic progression will suffer.
How is he at home? Doe she get distracted easily? Does he seem to be fidgety? Does he misplace things easily? Is he forgetful? Does he get distracted while watching a movie and can't get through it? If yes, he may have attention problems that his parents should discuss with his regular health care provider.