Do I have a right to be mad?

Six times in a row in 1 month my best friend has bailed on me. We make plans to hang out and the day of or day before, she bails on me... to hang out with her school friends that she sees every day during school and hangs out with almost every day after school. We don’t go to the same school so I only get to see her on the weekend. She saves Saturday’s for her boyfriend and usually we hang out Sunday but she’s bailed on me 6 times for her school friends. This has been happening for a while but started to become really bad this past month. I’m pissed. I hate how it feels like she’s choosing them over me, time and Time again. I know I should trust her and that she’s probably not purposefully doing it but I’m so mad. Is this a justified reason to be mad??