Does your boss really know what high school you went to? If he does, it is a high school in your area that he could know possibly know other people from and just not know it? If you changed your name and everything else I wouldn't worry about it and if he brings it up just deny it. Anyways, although it's embarrassing, it's not that big of a deal.
I don't want to make you feel bad, but I don't really understand why this is a big deal. If he brings it up (which he probably won't), I think you could easily laugh it off. EVERYONE I know is guilty of online stalking at times (AIM away messages, facebook, friendster, myspace, etc.) so I wouldn't feel bad about it. Plus, as embarassing as it may be, he might be flattered that you were interested in knowing his background
I dont' think its a big deal. Don't fess up because they might not even know. If asked just blow it off and say you were just seeing who was on there and typing in names and you could even name a few others in the office to make it sound harmless. Don't freak out over it.