I am currently a Head Boy at my school.
Focus on saying exactly what they want to hear and how you are going to do it, maybe 3 big points that you want to put into action that have been popular requests thus far. Additionally, appeal to their funny side, but be careful not to force a joke resulting in an awkward silence. Evidence that you're an all-rounder with the students also helps; describe how you talk to the different types of students in the school. Do something unique, be it a presentation or video to show the effort you will put into the role - prepare your speech beforehand, try using cards or your memory, rather than reading from a A4 sheet that might create a barrier between speaker and audience. Also, let your personality shine through, use some personal examples that your panel can empathise with to boost your persuasion. E.g If you want to improve school meals, talk about how you and the students hated last lunch's meal.
Good Luck