I am a Dad of two teenage girls...I'm not a Bishop, but they had both made different choices before they were 16 and decided to kiss some other boy.
As a very suspicious father, I seem to know my kids behaviors very well, and some things had lead to me asking each of them if there was anything in their life that had happened that was not in accordance to the Strength for the Youth program. I had asked them individually at different times, and they both had admitted that they had kissed someone. It was heartbreaking to know that they had chosen to do so. Something about being raised with certain standards, telling everyone that you live by those standards, then it all falls apart.
Anyway, my daughters are wonderful, honest kids that have made mistakes like the rest of us. I have a saying that I have raised all of my kids with..."Good people can make Bad Choices", that is why we are here right...to learn from our choices and to make as many good ones as possible!
I would bet it is as said above, that the Dad already knows there is something going on, maybe not to the extent that it is, but I'm sure he knows something. I have found that whenever I have taken something to the Bishop that I thought was new news, he had already known what I was telling him.
However, if he doesn't know, it would be your duty to report to him what information you have. Then it is his choice to do with it what he will. I had to tell the bishop once of a person that I had working for me that I caught him smoking...I felt like I was in grade school telling on someone, but he assured me that I was taking the proper steps for this person. I don't know what came of it, but I never saw him smoking again and he never said anything to me about it.