The Government Printing office
maintains a style manual that usually contains useful advice on matters like this. I just consulted the relevant chapter (
PDF here).
Because this guide is published by the
government printing office, all of its examples refer to federal government officials, so none of them applies exactly to JP's case. But I take it that "Mr. President" is reserved for the president of the United States. Having looked at the not-quite-but-almost precedents in the Manual, I think I was wrong in my first post. In JP's case, the correct way to address this gentleman (header and opening) seems to be this:
Donald Duck, DC
President, Mickey Mouse High School
[...]
Dear Dr. Duck!
[...]
I'm now pretty sure this is correct, especially since the header is consistent with the signature JP quoted. But I'll check my Chicago Manual of Style when I get home from work.