November maths resits

Hi eveyone. During the last year, we have had continuous math mocks, and I never got lower than a B in a single one. When the GCSE came around, I managed to get a D! I'm putting that down to exam stress. We had a test in my new sixth form, and again, I got a B. However, we did an online math test (not at all related to the actuall exam) and I got the equivalent of a D. They now want me to do a Saturday morning course, and 2 afternoon lessons after school to help me achieve my c grade (we were told to aim for a c in the exams) I don't want to do the extra courses, as I still have maths as lessons, and I'm still getting B in maths, but they won't let me resist my exam in November, and are making me wait the full year before resiting. I'm wondering if they can refuse to let me resist, or if there are any other ways of doing the November resits and getting it over with. Thank you.