What are your favorite songs to play? The audience has fun when the performer is having fun!
With older folks, I'm Yours by Jason Mraz and Ho Hey by the Lumineers are good clap and sing-alongs. That song from the Pixar short Lava is an easy 3-chord song and everybody sings along on the chorus. In your age group, I don't know if the Lava song is cool... or outdated already. Ask some of the other kids what they think. Ask the choir teacher what's popular.
When my son was in middle school (about 10 years ago) anything by the Beatles was retro-hip. Let it Be is a 4-chord song and I don't think it's possible for anyone to not sing along.