Grades are not just a measure of knowledge or academic excellence. They also measure how well students can deal with the rules and people in their environments. Even with horrible teachers, some kids get A’s. It’s a measure of resilience, emotional intelligence, judgement, discipline.
Sadly, some of my kids, though academically strong in terms of knowledge, intelligent according to IQ, great personalities, honest , and all other wonderful things, were lacking in discipline, focus, consistency, detail, etc to get the grades commensurate with their abilities according to test scores. So, yes, it is a flag if a high scoring kid doesn’t have good grades and it is a reflection on that kid. Now, if he had some really great activity, accomplishment that is a hook for the colleges to make up for that, it’s s different story. There are some truly gifted, talented, genius kids whose accomplishments transcend grades at school but those are rare. It’s got to be danged amazing to knock the socks of Harvard’s AO.