My sixth form says we need a D to do an A2 in that subject, and it's a grammar school sixth form *shrug*.
Different schools are different, maybe talk to someone high up to ask if you could go through if your grades are Ds, promising you'll resit etc, and they might listen, especially if you're close to the passing grade.
Otherwise you may just have to go to a college or something to sit them, as colleges in general are more lenient.