Just keep in mind that your focus should be clear, precise writing rather than using large impressive-sounding words. A large vocabulary helps when you find yourself struggling to find the right word for a certain situation, however, this is really the extent of its usefulness.
When you look at high scoring essays, most of the time you will be able to understand every word they write - they deliberately avoid using big, obscure words because the purpose of an essay is to communicate ideas, not to confuse or annoy the reader. The skill involved is being able to use simple everyday words appropriately in an essay. This comes with practice.
I recommend when you write your essays, highlight segments that you like the sound of, or that the teacher liked. You can then reuse similar phrases in later essays (so long as they are appropriate). This helps speed up the writing process and means less time searching for the right words to use because you've already explained something similar before. Once you've written out the same argument 10 or so times in different essays, the process becomes more and more autonomous.