Boyfriends ex trying to break us up?

Last summer me and my bf broke up cause of our parents & we got back to gether this year cause we dont care wat they think now and his ex that he dated years ago is now a freshman and she is constantly tryin to talk bout how they were together & stuff. Well then she goes & said thatthey went out over the summer & he broke up with her to go with me but somethin was tellin me she was lieing. Asked my bf bout it and he said that she was lying that it never happen & he didn't lie since I can tell when he does. Few months ago she tells me while we was getting ready for band that she hopes we break up & then she said she was kiddin. Well sheconstantly asks me when I'm n a bad mood or daze out if we hav broke up. We hav been thinkin that she is tryin to break us up & she talks bout how she still likeshim & all to me. Well now she is going out with his best friend trying to make him jealous & to get info bout us & get closer to him. My bf told me that his best friend told her that we think she tryin to break us up & now she is trying to turn everything around sayin that he is tryin to break her & hisfriend up. That's not true cause he don't really care bout his friends love life & he dont go & tell him wat to do. I hav also been told from people who known her alot longer said she's a manipulative b**** what can I do to stop her cause if we cant then the last 2 years of high school will suck pls help