Replying to Jib Academy or Semi Formal

Alright so I just found out that my school's semi formal is the night before Jib Academy. I really want to do jib academy seeing that its at my home mountain (Holiday Valley) and I've really been pushing myself hard this season to learn new stuff. BUT my girlfriend really wants me to go to semi and she's going to be pretty pissed if I tell her I cant go because I want to go skiing. My parents leave every friday night to go to my place at Holiday Valley and they said that I have to pick one or the other. So which one should I pick? I'm leaning towards doing Jib Academy but I'm not sure yet.