No, we Prefects don't cast that particular spell on each other. Remember, only the Head Boy or Head Girl has the authority to cast it (and generally if someone has demonstrated the type of trustworthiness that leads to them being picked as a Prefect in the first place, they'd be unlikely to do anything that would necessitate such a punishment). You may have entirely sensible reasons not to want to be a Prefect (such as wanting more time to revise without having to work around a patrolling schededule), but the fear that you would be required to cast 'Crucio' needn't be one of them.