Warning: This is a personal, non-professional observation, based on a small sample size (my high school).

I come from a pretty small town. My graduating class consisted of only about 120 people. I knew just about every one of them pretty well, having grown up with them. Not many people decided to go to college after high school from my class, less than 25%. Knowing these people, I can say that nearly all the people that did go to college seem to be smarter than most of those who did not. There are a few exceptions, of course, but in general, a person's level of schooling tends to be a [u]good overall estimate[/u] of intelligence where I come from. All the high school drop-outs that I know were ignorant trouble-makers, not genuises.

Personally, I believe that I've become more intelligent due to my university-level schooling (I'm only half-way done with my bachelor's right now, however).