Why do people always think they can improve.
If you get a C at GCSE it is unlikely you will progress to a good AS and eventually A2 grade.
If the school wants a B, then you could go to evening college, but it will be a lot of hard work. If you did not try at GCSE then it is unlikely you will try any harder at AS, especially at evening class.
If you did try a lot at GCSE, then you got the best you could at a C (which is fine) but then I would advise you not to take it onto A level because a C at GCSE normally translates to a U-D grade at AS level.