Originally Posted by
TypoNinja
That's pretty plausible, I'd buy that.
Though from my point of view, I'd say that Social class is the limiting factor. D&D approximates a medieval tech base, and usually Feudal societies as well. The vast majority of the population then is the peasantry, and as such are mostly of the farmer/craftsman profession. The vast majority of whom have not the time to learn to read and write. Illiteracy among the peasantry would be quite high. Going to schools is something that is for rich peoples children, a Magic school would be like a university, even more of a rarity fior this class of people.
Even then, they still actually need the requisite intelligence. I'm sure there are oodles of students who can magic missile, but if 10 represents human average, then even students who can learn to fireball at a 13, are a cut above. There will be plenty of graduates who know some magic, but not enough to make a living, they will then largely ignore magic for more mundane careers.