How can I do well in a writing exam where there's so much room for mistakes?

Writing language exams can be difficult - you need to have grammar, language and message in mind as you write, but I found that practice was the trick. Aim to write as many practice exams as you can (I recommend one a week), and do time yourself. You'll be training your mind to process the question, plan a response, write and proofread it in the time allocated.
Secondly, familiarise yourself with the criteria! Read it before you sleep, when you wake up, and on your way to school - I cannot stress the importance of the criteria enough. Finally, you should be acquiring a good level of vocabulary to use on your final examination, so make a word bank and keep it updated as you go through your lessons.