At all costs avoid using fancy words if you are not absolutely certain what they mean. You are being graded on substance, not style. Resist the temptation to use legal or Latin terms if a simple English word will do. But do be sure to cite rules and cases where relevant, and to explain the relevance. A related, and also frequent, error on law school exams is a failure to be sufficiently specific.
Legal writing is a form of technical writing.
It thrives on precision and often on detail. I suggest that you always avoid the passive voice because it allows you to neglect to specify the identity of the actor in your sentence. A third, surprisingly frequent, error is forgetting the point of view you are supposed to take in answering a question. There are differences between these roles. For example, a judge is quite likely to be concerned about judicial economy.