I would hope that he would tell me about it, rather than feeling the need to go to a teacher.
I can see your point Jack, about the teacher thinking he was 15. If that was the case, he certainly should have reported it.
Though, in all honesty, I don't know whether the police could have definitely followed up on it. And I'm not sure if the man who was having sex with the boy knew his age either.
It seemed a mostly victimless crime to me. The boy was of age, and had sex with consent. Even if the boy was 15, I am leery to demand the teacher turn him in. Kids that age have sex, and I would assume that the teacher knew 1) this kid was going to have sex eventually and wanted to stress the importance of protection and 2) thought that if he was coming to him, the teacher, that maybe the child had reasons for maintaining privacy. (Ie. he didn't want to be outed to his folks)
The above paragraph, of course, is mere speculation. It could be that this guy just has really poor judgment too. None of us can say for sure.