What should I do to solve this situation?

Ok. So this is what is going on: I was elected as a prefect at school, but my teacher said that all prefects are required to partcipate in an activity, which I am not available to do so. However, both of us forgot about that until today. That school activity will begin on Tuesday!! What should I do? Should I just ask a friend to go in my place? (We're having public exams this year and they may not accept) Or should I take a risk and meet the teacher responsible for the activity and just tell her the truth? What should I do?