1. Intelligent people often come from intelligent parents, who will stress to their children the importance of schooling.
2. Schools often have special programs for "smart" kids to challenge them.
3. Similarly, if the school doesn't present such an opportunity, there are scholarships, places like Melbourne High/MacRob etc. which foster highly intellectual students.
4. Many smart kids have massive egos that come with their intelligence, and in the event that there is another smart kid in the class, they will strive to compete (and beat) them, hence more application in schoolwork.
5. Many of the best performers at school are the ones with some sort of social disability - hence, the "weirder" they are the more likely they will apply themselves (as often they don't have anything else to achieve in).
I'm not saying this applies to every intelligent person, but I am saying it does to a lot. I can prove it because I'm Asian, know a lot of smart Asian kids (and by that I mean I have almost 1000 Facebook friends!!!!!1+shift) and go to a school that is supposed to represent a high level of intellegence within Victoria (median ENTER score of 90).