I heartedly disagree with bullet007's perception that you should use similes and metaphors in your papers, save for a few exceptional cases. What you are writing is an essay, not a children's story littered with pointless comparisons such as "the bird was like an angel who descended from the heavens" or "his knowledge was abundant, an encyclopedia containing even the rarest of information."
Frankly, University English WILL kick your ass unless you put in a boatload more work than in High School. For example, I handed in an essay for an English Major introductory course that would guarantee me a 90+ in High School, only to walk to away with a 65.
The key to doing good is two things: First, be as direct as possible. This advice doesn't change regardless of whether the word limit is around 1500 words, or 5000 words. The professor doesn't have time nor the patience, as I painfully found out, to deal with rhetorical questions or non-relevant information. If you find an obscure piece of information that is brilliant and insightful, go ahead and use it, IF AND ONLY IF it's relatable to your thesis - ex/ Dido from Virgil's "The Aeneid" may have been a historical character who couldn't have possibly survived in the same era, but that has absolutely no relevance to how significant a role she played.
Secondly (and this here is probably the most important point), EDIT. I can't tell you how ridiculously good your classmates will be in an English course, but it's best to assume that they write at an equivalent level of a popular blogger or a minor author. And since your essay will undoubtedly be compared to everyone else's essay, you want and need your essay to be as polished as possible. After you finish editing once or twice, get a friend or go to your university's writing aid center to read over your essay and determine the weak points. Take those weak points (hopefully it's not a key content filler) and get rid of them. Refill it back in, edit once or twice more, and you should hopefully have a paper around 80%.
Lastly, if you have time, read the Elements of Style by William Strunk Jr.. This very useful book will smash most, if not all, your bad habits in writing and clarify any common grammatical misconceptions. And over your future writing career in university and beyond, it'll end up as the most worn and overused book in your library. Cherish it as your muse.
Best of luck.
