You are right that shorter sentences can seem simple, but they aren't easy to write! You have good control over your clauses and sentence structure: don't be afraid to try a mix of short and long sentences, whether in academic or other writing.
A good place for shorter sentence length is the topic sentence for each paragraph. Have one crystal clear idea in it, and then develop the idea in the longer sentences following it.
Looking forward to your next essay!
Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts.
William Strunk, Elements of Style, http://www.crockford.com/wrrrld/style3.html#13