Hmm, I think that using your actual experiences from your life is perfectly valid, and that you really don't have to worry about how you portrayed those bullies in the future of your novel.
IANAL, but assuming that these incidents from your life aren't really public knowledge, there's no reason that anyone could claim that you're libeling them. There'd have to be a clear connection between the characters and themselves (and you've changed the names and I assume the descriptions and the incidents themselves somewhat). To do that, they'd first have to admit to being those bullies in real life, which really would do more damage to their image than any fictional portrayal you might put those characters in.
Secondly, think of how you would have handled this before today's ability to track people through social media. You would have no idea how they grew up. You would have no connection at all to them except through your memories.
Finally, I say that if they do sue you, sue them right back for mental trauma when you were a kid. I mean, really, what defense would they have if they came out and admitted it?
I might ask this one more publicly on AW (in the novels or Roundtable section), since there are actual lawyers here.