This really depends on your exam board. As far as I know teachers are allowed to read your work and give general advice but not any specific pointers.
I would suggest starting off with the basic three tenses and making sure the foundation of your ca is secure. Make sure ideas are fully developed and flow on well from each other. This will in most cases guarantee you a C and even a B grade.
After that, look at including the conditional, simple future and imperfect tenses.
Use different pronouns.
Learn adverbs, adjectives and connectives, but don't overdo it.
Use idioms.
Vary how your sentences start.
If you are confident with this you could explore the subjunctive tense, but I suggest you focus on the stuff above first.