We do prefect training in-house and a good activity is to get them (in teams) to design the perfect prefect - we normally give them flip charts and marker pens. It's good for encouraging discussion and it's quite interesting the different perspectives that they have! We have sessions on school-related issues (e.g. child protection, sanctions, key SMT/management roles) and on soft skills (e.g. commmunication and presentation skills, leadership styles, organising a team). There's also some "how would you deal with this situation" type activities.