In most cases, professors will want you to use a type of clear, no-nonsense language, known as "Academic English." Academic English is the standard form of written communication for reports, research papers, and other assignments. It's not flowery, or filled with big, important-sounding words. The point of using Academic English is to get your point across efficiently and elegantly.
Here are some great websites that can help you master the basics of Academic English:
Academic Phrasebank - this is an excellent source for instances when you need a new or more compelling way to present your case. The Academic Phrasebank offers suggested phrases for you to use to make your points, and it covers a wide variety of common problems you might encounter while writing. It's indispensable!
Using English for Academic Purposes and Academic Writing, from the Purdue OWL - these two websites present more comprehensive information on the topic, as well as explaining how to handle different types of circumstances and assignments.
What is Academic English? - a basic introduction to the conventions of Academic English.