I prefer Ms. personally, and if I don't know what a woman prefers to be called, I use Ms. by default. That said, I think that it's far more disrespectful to refuse to call a woman by her preferred title (assuming she's not demanding to be called Dr. when she's not one, or some other title she hasn't earned). If a woman prefers Miss or Mrs., that's what should be used.
However, if you know your professor will lower your grade for using Mrs. (which is utterly ridiculous, and I say that as a proud feminist), you should probably use Ms. It's not worth risking your grade.