During my time at Dartmouth my Flight was divided into two sets for academic studies; Graduates and Science based A-Levels and those with non-science based A-Levels/O Levels. I think there were 24 of us, 12 pilots and 12 observers and the split between the two sets was about 50/50. After the ensuing flying training 5 of us actually got our wings and went front-line, whilst the rest were chopped or resigned. All of the 5 that qualified were from the non-grads set, which either proves you don't have to be academically gifted to be aircrew, or the grads found it all too boring and easy, but as with all statisitcs you can draw any conclusion you like.
p.s. One of those that was in the thicko's set is now a Commodore, but you wouldn't have spotted all that potential back then!