I would focus on exam skills, look at the marking criteria for the examining body your school does. Focus your intervention on showing them what a C grade piece of work looks like and how it differs from theirs. Show them exam skills and time management. D grade students often lose out on a C grade because they miss out a question because they have run out of time. Do lots of timed questions. Model a question (only one per session) show them how to answer it relating to the mark scheme and then let them try it themselves. Feedback honestly but so that they can see how they improve. They really do need to know exactly what they are doing when they get into the exam. Hope this is useful for you.