maybe if you hadn't made bad grades this wouldn't be an issue?
I mean, yeah as a kid it seems really forgeted up and stupid, but all you have to do is just prove to him that hey, I can get perfectly okay grades.
I suggest just trying your hardest and after a week or two confront your dad in a respectful manner just asking for a compromise, if you've been doing your absolute best then he has no reason to keep you grounded, at that point I would say he's just being a douche