Whether you would receive points off for using a conjunction is entirely dependent on the instructor's grading system. Technically, conjunctions are intended to join two sentences together; therefore, using a conjunction to begin a sentence is not grammatically correct. However, we use this format all the time in speaking and you will often see it in all kinds of writing as well. Personally, I avoid using a conjunction to start a sentence in my formal writing, although I might use it in an informal email or letter for emphasis and to make my tone sound more friendly and casual. I discourage my students from using this sentence form in essays because they tend to overuse it, and because most of them need to practice writing in a more professional manner. What is most important is that you understand that using a conjunction to start a sentence gives your writing a more informal, casual tone. If that is appropriate to convey your meaning, then you should use it. On the other hand, if you are trying to create a document which makes you sound authoritative, you should avoid that construction. The simple answer to your question is that you need to ask your teacher about their standards on this issue.