Okay, so I have a really good friend, but she left several friends in the lurch today...?

Okay, so my really good friend, her name is Alysun, is the editor for my yearbook cl in my school. We have not been very successful in selling yearbooks this year though, and our teacher is getting mad. Today, she ditched our Yearbook meeting at lunch to go and be with her new boyfriend, who is an absolute pig. Last year he tried to beat me up with no provocation. The rest of us took the heat from the teacher for her, and had to listen to her rant. However, Alysun still plans to ditch again next week. What can I do to make her realize that we will not tolerate this? We will not be her scapegoats again. She was the main one who wanted to do the whole yearbook in the first place, and this will be the first one ever done for this school. I don't want to upset her too much, especially since I am going to her Birthday Party on Saturday. What can I do?