Tell her that it s cancelled.Tell her you don t want to go, ur gonna stay in class and get caught up on work. (if ur school is the type that leaves students who don t attend at school with a substitute.)Tell her more Parents volenteered to chaperon then they needed, so they ve picked the number they needed are all set.Or try being honest upfront just ask her not to go.
Tell her that it is canceld. I have no other idea. Good Luck! :)