Is it worth fighting over this?

So me and my boyfriend are very happy and in love but there's something that always gets to me. So back then my boyfriend messed around with a couple of trashy girls.
He slept with all of them but one of them is a girl I despise so much because of previous problems. They agreed on being friends with benefits and she asked to come over to have sex with him. She's currently in my classes in high school and every time I look at her the thought of him and her gets me so mad. She knows I dislike her. Here comes the best part: one night me, her and a couple of other friends are on our way to a party. She decides to call up my damn boyfriend saying oh you should come to this party, I miss you, I wanna see you, etc. when I realized who she was talking to I was pissed. So when I told her that's my man she was like omg I didn't know I'm sorry blah blah. Ever since then she stopped talking to him and they haven't had any contact. Literally every time I see her I feel this rage for some reason. She's the biggest hoe in our school. He claims he hates her and doesn't like her and was just a one time thing in his past but should I believe it? I know it's his past but I can't seem to let it go.. She has interfered somewhat in our relationship he hates talking about her or when I mention her because we end up in fights and arguments.