15 yr old daughter misbehaving and we cannot seem to get control of it...can u help?

Ok so our 15 yr old daughter asked if she could go to PROM with her best friend at another school because that girls boyfriend moved. Mom said yes and agreed to pay the ticket. The girl offered to buy our daughters dress. The girl is a senior and my daughter is a freshman. Since then my daughter has been caught ditching excessively and I was informed this week she is failin all her cles. She has lied to us and the teachers over and over. We just got reminded the PROM is tomorrow. Her mom says let her go because its not fair to the other girl. I say NO WAY..and she can work to earn the money to pay her back. A: What do you think is best? ...and B: she syas she is not going to school because she is afraid of all the violent fights...if that was true wouldnt she have said so before she got caught? What do we do...this is like and a b maybe c question...sorry