That's exactly what happened to me (and a lot sooner than 25 years after high school). On graduation from Gymnasium (would be 12th grade here), my friend and I happened to run into this guy who would pester us in 7-9th grade. Worse for my friend than me, and nothing like broken fingers, but just random physical violence every now and then. We hadn't seen him for a couple of years, so we said hi and started talking and he says "I'm sorry I was such an asshole back then." It increased my respect for the guy immensely.

So yeah, you grow up and as part of that you realize you did some shitty things. So if you decide you really want to communicate with the people you were mean to, the first thing you do is straight up apologize. If they don't, I think his incredulousness is justified.