Thanks for everyone's responses. I have decided not to send the letter at this time -- and perhaps not at all. I did think about the possibility of a defamation suit, but I'm prepared to defend one as I have kept several artifacts from that time including letters he wrote to me, my high school yearbook (and what he wrote in it), etc. I also considered the possibility that he might interpret the letter as blackmail and go to the police, so I wanted to make clear that wasn't the purpose.
Ultimately, however, I'm not sure what the purpose of the letter is other than to vent and put this guy on the defensive. I'm not convinced those reasons are good enough. I read a story yesterday about a teacher who managed to keep his job after grooming a student, and it really set me off. (I've posted the story to the U.S. & World News section of this forum.) My letter was in part a reaction to that story and the sense of injustice I felt reading it. I think just writing the letter and having a good night's sleep has made some of the rage dissipate, so I feel less need to lash out at this guy right now.
Anyway, thanks again to everyone who took the time to reply.
That's the worst, I think. When the secret stays locked within not for want of a teller, but for want of an understanding ear.
-- Stephen King, "The Body" (from Different Seasons)