Well, obviously this is a form of rebellion, which is quite normal for a 14-year-old, I believe. So here's my suggestion:
1 - a dose of their own medicine: If they talk to you in french, simply say "sorry, I don't speak French, do you speak English?" and leave it there.
2 - If I understand correctly, this will have a negative influence on their school marks, right? So, I would say let them be. Let them loose the marks, and then you can give them a hard time about doing badly in school. (I assume that at 14 they are a few years away from needing good marks for college entrance etc, so it won't cause permanent damage to their records.)