My 13 year old daughter is changing what to I do?

My 13-year old is driving me insane!! She is a great student and has high expectations. She was recently accepted into a private school earlier this year but this last semester she has really declined...I mean going from a 97 in excelled math to a 65!!! I am so livid, I don't know what to do. We have a great relationship and in the past month she approached me saying that she had a boyfriend. I was open with her in conversation as I wanted to keep that door of honestly open and we had a great talk. She understood me and said she really felt I was right and that waiting to date someone is a good idea because she has to stay focused. Well that was a crock! Which is why the grades have declined. I have cut off her texting completely, talked to this boy and explained to him their BF relationship is over and no they can not even be friends because to me that line has been crossed. I do not judge anyone but this boy's mother has 8 children including the one that likes my daughter including him. But he is so stuck on waiting for her to be able to have BF's!! yes Psycho! Anyway I just don't know what else to do. we talked for 2 hours or I should say I talked. I did not scold, I even gave her issues I had in the past with boys and females as well - as far as peer pressure and get nothing back. What now?