Ha! Got you beat -- I've not been to a single one. And on the very few occasions I've run into an old classmate, they have never remembered me being in "their" class. I was a wallflower back then, I guess. Either that, or I am very forgettable.
I went to one a few years ago, but it wasn't mine. All of my friends were 2 years older than me. No idea how it happened, but I was invited to that reunion. My 20 year is this year, I can't remember a single soul that shared my grade.