Creating good academic style in your essays is partly a result of increasing 'fluency' in this sort of writing, but also the result of taking time to re-draft and edit your work before submission. Obviously, you do not have the time in an exam situation to redraft your writing and polish the style very much. Nor is it a priority for the examiner in marking exams. It is understood that the exam essay is a different sort of text than the supervision essay or dissertation, and as long as the writing you produce in an exam is clear and broadly appropriate, it is not judged according to the same standards as a supervision essay. The need for precise referencing, for example, is not appropriate in an exam, which tests different skills. However, you will not have the opportunity to clarify your meaning for the examiner, as you would in a supervision, so your writing needs to be clear and unambiguous.
In adapting the strategies for improving your academic style to an exam situation, you should however try to build in some time at the end of each question, or at the end of the exam, to go over your answer and correct any errors or areas where you have not expressed yourself clearly or in as much detail as you could. This is good general exam technique, but could be particularly useful in amending some areas of style. Aspects such as clarity, accuracy and structure should be a higher priority, however. Practicing exam writing as part of your revision will help you to write in an appropriate style under a time constraint, and allow you to identify any areas of style that might need tightening up before the exams.