You'll be allowed to do resits at Sixth Form, yes. If you got a B you'd probably need to converse with the head of sixth form and see what they think, although they'd probably let you. You would be doing the November retake yes, but just remember that Maths GCSE lessons will be incorporated into your timetable in addition to your A-level choices. You wouldn't have anymore than probably one or two lessons of Maths a week (assuming you're on a two-week timetable), but if you're on a one-week timetable you'd probably have one or two hours a week.