In trouble for Truanting?

Well on Friday we were suppose to be doing a charity walk with our form at school. (charity! an optional activity were you do not need to take part if you don't want to) Our teachers said we HAD to raise �30 each and they went round our class pointing at us all say �30 �30 �30. So we didn't turn up to this walk as they said we had to do it, they never said if you do not wish to take part just come in to school like normal. So yesterday they pulled us all out and said we have an hour detention, a possible �50 fine and our prefect status has been removed. But I honestly think there the ones in the wrong. Can someone tell me where I stand in this and what I can do?