My mentor professor has threatened to take authorship of my work away from me because of a disagreement. Can she do this? Is this plagiarism on her part for not including me as an author? I wrote the original paper, before editing. She never provided a syllabus and kept tacking on more and more things for me to do. I told her she was taking advantage of me and was told to go to the head of the department. She now says that I can no longer get a grade above a D, even though I am an A student and won an award for my work at a symposium. I have no idea what to do about this situation.
Usually a university will have a policy specifically about publications and student projects. I'd recommend beginning by finding their policy. But in general, yes, she can do this (all personal ethics aside). Assuming she does not use any substantial part of what you wrote, it is not plagiarism. As a student project, your work is generally considered like that of an RA (again, this is legally, not morally), so you have no requirement to be listed as an author of the paper. Assuming, of course, that anything you wrote is not being used in the publication.
My recommendation would be to try and discuss it with your previous advisor, and if they are not willing to discuss it, or you cannot reach an agreement, ask your advisor if they will get the chair of the department to mediate (she'll be less offended than if you just go over her head). If she says no to that, discuss it with the chair yourself. But be clear on exactly what you want to get out of it. Whether that's a chance to write it yourself (which doesn't legally make you first author as a warning), or merely inclusion as an author on any final publication. Always keep your end goal in mind in all conversations.