You volunteer to help, if you can of course, with the fair/fete at your school? I would say, put your name forward, be clear about what you can do/time you can do, and finish when you said you were going to.
But please so pitch in, even if to run a stall for an hour. Maybe the fair looks chaotic, maybe a few stressed people don't necessarily seem friendly on the day! But please don't let it put you off. Children love getting to do something in their school that isn't actual school, and the money raised does help. Especially these days.