Hmm, I've been seeing this attitude floating around that somehow grades are so closely associated with intelligence and success. It's a comforting thought if your grades happen to be good, but it just isn't true. Even though grades are the standard "measuring stick," grad school grades aren't even looked at for most post-doc work. A friend of mine got a postdoc at Caltech, and they didn't even ask for his transcript.
Many of the smartest and most accomplished people in history have been high school drop outs. Dropping out of high school probably isn't a good way to achieve your goals, but it might be, and it doesn't necessarily say anything about intelligence. There is a lot more to take into account.