The content of your essay is more important than the style of your writing.  But you should be aware that content and writing technique are closely linked.  You may know* the material, but if you cannot convince the reader that you know, your grades will disappoint you.  There are four general standards which must all be observed:

1.  Your writing must be clear:  Be sure to say exactly what you mean.  It is not sufficient to hint or suggest your meaning.  You must state your points explicitly so there is no doubt about your meaning.  Students often ask, "Couldn't you figure out what I meant?"  It isn't the reader's job to guess your meaning.  It is your job to say it clearly.  Even when I suspect that a student knows an answer, if it is not clearly stated, I will not give credit for what is not said.

2.  Your writing must be unambiguous:  Although this is closely related to clarity, it is so important that it deserves separate mention.  Your writing should not be open to multiple interpretations.  Statements that are too general can cover too much ground.  Poor grammar or poor word choice can confuse meaning.  You must communicate your ideas so there is no doubt about your meaning.

3.  Your answers must be complete:  Partial answers deserve only partial credit.  To get full credit, you must answer the entire question, not just a part of it, and certainly not some other question (like the one you studied for).  Multiple-part questions require multiple-part answers.  Giving a complete answer to the specific question asked demonstrates your mastery of the material.

4.  Your answers must be accurate:  Being clear, complete, and unambiguous doesn't count for much unless you are also accurate.  Silly mistakes or oversights can rob essays of their accuracy.   (For example, writing, "Smith would agree with Jones.", instead of,  "Smith would disagree with Jones.")  Unless you re-read your essay for accuracy, you run the risk of letting little mistakes rob your writing of its intended meaning.  Take the time to review your work for accuracy.

* Passive Understanding vs. Active Mastery:  Students sometimes confuse passive understanding with active mastery.  Because material makes sense (passive understanding) when they read it, or when it is discussed in class, they think they "know" it and are disappointed when they earn a "C".  Active knowledge and mastery require not just that you understand the material when someone else speaks or writes about it; they require that you, yourself, are able to clearly and accurately explain what the material means and what it implies.  Just as passive understanding of a word does not guarantee that you can use it correctly, passive understanding of a subject is not the same as knowing it.  Passive understanding earns a "C", at best.  Active knowledge earns a "B".  Mastery earns an "A".