Should I be upset?

hey guys. I've been with my boyfriend for a year. I graduated from high school last year and he graduates this year. there's a girl in his grade and we don't like each other at all. She's all over him. She try's adding him on snap, texting him and all that. She has a boyfriend but she's just one of those girls who cheats and has a thing for taken guys. If she sees my boyfriend and I together she won't say anything but as soon as we're apart in public, she walks right on over and starts trying to touch him and all that. All our fights are because of her. My boyfriend doesn't know why I get mad when he's with her. When we were at prom, he forced me to leave (he left with me obviously) because "he couldn't stand the pressure of her and her group looking at him." What's that mean? And do I have a right to be mad? I'm so confused as to what I should do.