Hi
I wasn't a prefect, i was actually head girl and back at my school you didnt apply for prefects the teachers choose you based on your performance over the years. you applied to be head girl.
i can tell you what the teachers looked at.
speak about your involvement in after-school activities, and activities you're involved with outside of school is a bonus, tell them about your consistent grades over the years, have a teacher endorsement (so when they are looking at the applications you have someone to vouch for you in the room) and tell them about your love for your school. i mean thats why you're doing it, to keep raising the standards set
i hope this helps, with this lay out we had a close race for head and deputy, in the end there was a tie for deputy so we had two.
best of luck to you