I'm a few years younger than Romney, and I certainly don't remember everything I did in high school. It's quite possible he doesn't remember holding down a younger boy and forcibly cutting his hair to make him conform to Mitt's standards of manliness. It's really very possible.
But for that be true, it would have to mean that he had no emotional investment in it. That it was an ordinary day full of ordinary things.
Look, hijinx and pranks are one thing. But this was neither. This was cruelty. And for Romney not to remember it is disturbing, but of a piece with other things the many does and brags on doing. We all know the story of Seamus, but have we forgotten that it came out as a testimony to his toughness and no-nonsense way of doing things, from his supporters? His total inability to empathize with the people whose lives he ruined at Bain is another one.
So, yeah. I can believe he doesn't remember it. And that's worse than him lying about it.