First, because of the legality, I would not encourage it as the father of this young lad did. After the fact, I would ask my son exactly how it happened, and how he feels about it and go from there. If it was a mutual thing I would probably tell him to keep it quiet, meet up with the teacher, and give her a warning that the next time, it may end poorly for her because it is illegal and frowned upon by society. If it was a situation of against my boy's will, then hell would break loose.
Now, if this were my daughter, it would be an entirely different ball game. YES, I know, it's a double standard but it is what is it so let's agree to disagree now.