Ask for the paper back and look through where you went wrong. Get help on those areas, either by using websites on your own or having a tutor/friend that can help. If you're not allowed the paper back, ask the teacher where the class generally went wrong and go over it in class.
Speak to others that sat the exam as well. If you're all getting B/C/Ds in your exams and you all did enough revision, then it's something going wrong with the school and not with you.
Pinpoint the problem ASAP and move on. Time is precious now.