i think if you live in a small town and you went to school with them for 12 years, you will be more bonded with your graduating high school class than with your college mates...just my 2 cents
i have kept in some contact with both and it's hard to say*who i am closest with
I think, as you say, it's a toss up. I actually did grow up in a small town (!6000 people), though I didn't go to school for the full 12 years there. I showed up after first grade and skipped two grades along the way, and so was only with the same class from 6th through 12th. I made some very close friends, though my most enduring friendships from that period were with a couple of people in classes above me.
My situation might be different from others' because my family moved away from that town almost immediately after I graduated from high school. For that reason, I haven't been back to the town where I grew up in six years or so.
That said, here's what I think the difference is between high school and college. In high school, especially in a small town, you make friends with who's around. The population is often a bit more homogeneous as far as interests go than a larger town or, say, college. My university, for instance, had more undergraduates than my hometown had people. I was exposed to a lot more people who shared my interests, and a lot more really interesting, bright folks, just from the fact that people were coming from all across the country. What's more, people are actually starting to figure what it is they care about when they get to college, at least more so than in high school. I think that opportunity to find people who strongly share your interests (instead of just your zip code) adds a lot to the college experience.
I see where you're coming from, though. Twelve or more years with the same group of people forges strong bonds--time will do that. But I think that for a lot of people, myself included, the four short years of college involve interactions that are so intense and concentrated that they are worth much more than the many years of grade school. Like I said before, you live with these people, and that forces you to work out your differences and get to know each other in a pretty intensely personal way.
But in the end, I'll waffle: What's true for me isn't true for everyone, but the main thing is to find people on whom you can depend and in whom you trust somewhere along the way.