Another teen that dislikes his parents rules. What to do please?

Well, 1st quarter I finished the class at 100 percent. But a lot of crap went on in the second. My OCD and ADD began acting up again, but I wouldn't tell anyone because they would punish me severely for it, even though they know it was serious, to where I went to therapy. Plus a lot of crap in personal life went on, and I finished with a 79 percent for second quarter. I got a 100 on the semester final, so I finished with an A for first semester, which is the only one that counts. But ,y parents said that when that report card comes back with a C for quarter, I will be grounded untill the end of summer (they will do it too, they did back in third grade). Well, I had two assignments that I turned in late at the beginig of this quarter, so they will be 50% (right now, since the teacher hasn't graded them, they are 0%). By the end of the week I, from other stuff, I will have an A again. But now, since they said I saw that coming last quarter, even though they forget that there were tons of home issues, they said if the assignments, regardless of my overall grade, are even 50%, I will stay grounded until 18 and have my stuff sold. From my sister, I know they will hold to this as well. I really am putting my foot down and trying now, but those will NEVER be higher than 50% because they were late. So I'm screwed. And this is a class that is both honors, and a class that I am not supposed to take until next year. It's not a cakewalk. I don't know what to do now though.