I've been threatened with a lawsuit over a grade

Right now my head is spinning and I'm literally shaking. I don't even know what to do or who to turn to. I'm a highschool English teacher and at the end of every grading period (semester) students have a final exam that's pretty much a mile long essay (can be around 3-4 pages). <br><br>Well one of my students who will be graduating this year isn't very bright, and she knew that she was going to fail my class. On the final her and some of her other classmates decided to cheat, so therefore they all received 0s. This 0 caused her to fail. Now, she's going to summer school so she can still graduate with her class. If she fails summer school she won't be able to graduate. Well she came to my classroom (and I was there even though it was summer vacation) and threatened to sue me for "ruining her senior year." <br><br>She told me that her family is very wealthy and they'll be able to hire the best lawyers, and she'll definitely win. I'm very scared right now and my principal doesn't even care. I really need some advice on what to do. I don't want to lose my job over this.