I'm a mom and I have kids your age. My 13yr daughter got an F in 2 classes and I grounded her too. I asked her if failing was due to the hard subject and she said no. She told me she failed because she chose not to turn in her homework. In her school, final grade cannot be greater than homework grade. Since she got an F in homework, it doesn't matter what her final exam grade was.
She too is grounded until her grade goes up. All my children attends an accelerated school which teaches 2 grade level higher than public schools. She has 4 hours of homework every day and even on weekends. She is smart and I know she can do the work if she wants to.
So ask yourself, are you failing because you were not trying or is the subject hard to understand? If it's due to laziness, then suck it up and do your work, but if it's because you don't understand the subject, then ask your parents to get you some tutor help or afterschool help from your teachers. Good grades only come from hard work.
I don't expect my kids to get straight A's. I only expect them to try their hardest. If they try their hardest and still get a "D", then that is fine, but if they didn't try and get a "D", then they better be ready to suffer the consequence.
This is the advice and question I ask my kids. Do you want to be the waiter at a expensive restaurant or the customer sitting and being served at the expensive restaurant. If chose to be the customer, then you better work hard on your education. If your satisfied with being the waiter, most likely, you will regret it when you turn 30.
I suggest you do some soul searching and then talk to your parents. Sorry for being such a mom.