The nutrition rule doesn't always apply, though. I grew up dirt-poor (as in, one low-earning check away from living in the street), and while I made shit for grades in classes that weren't math (or programming during freshman year in high school), my teachers all noted that I was actually quite intelligent, I just had problems with organization and homework because I've got ADHD. My diet mostly consisted of cheap, bulk, processed foods for much of my life. I think a large part of it is the parents not actually encouraging their children to be intelligent in the first place, or at least find something they're good at. That's something my folks did right, even if school did leave me with some...very deep psychological scars that have yet to fully heal and retarded my emotional and social development until I was 23.
But, I do agree with Reika, a lot of is the fault of society and popular media. While "nerds" are seen as trendy nowadays, actual, functional intelligence is largely shat upon. Well, they can envy us all they want; somebody has to clean our toilets, it might as well be the brainless ones.
That first part is very close to what my childhood has been like, the only things that are different are the skills, the mental issues, and how much money came in (its close to it though.)