There are, indeed, regrettable incidents one might perpetrate or participate in while in high school, or even in a tony prep school. It may be an isolated occurrence from a long-ago past, and we are certainly talking about nearly 50 years ago.
HOWEVER, and here is where I diverge from Press (who is usually pretty smart), if an incident from long ago isn't isolated, and echoes of it show up in a person's later life, then I think we can fairly say that rather than an aberration, the behavior is part of a pattern. Is there anything in Mitt Romney's subsequent life that indicates his foray into the forced tonsorial arts in prep school was part of a pattern of behavior? For example, has he ever taunted and mocked people subject to being drafted and sent off to war when he himself was safe from conscription due to his religious obligations? Does he have a subsequent history of oppressing weaker creatures unfortunate enough to cross his path? Has he enriched himself at the expense of others who were powerless to stop him? Does he flaunt his exalted financial position and evince no understanding whatever of people who are not as well off as he? Does he consistently try to avoid responsibility for errors or mistakes of his past when his words and actions turn out to be grievously wrong? Is his sense of personal entitlement beyond anything a normal human being could recognize?
If the answer to any of these questions is yes, then the hijinks of prep school might be germane to the man as he is now. If the answer to several of these questions is yes, then you can be pretty much assured that as the twig was bent, so grew the child into the man. And if the to every last stinkin' one of these questions is affirmative (and plenty more beside could be so asked and answered), then you're dealing with a very important component of a man's character. It's up to the voters to decide whether they want such a person in high office, and it's up to pundits and observers of the political scene to be sure the paint as complete a picture as possible of such a man.