Brekelefuw

If they are beginners I would suggest doing a blues in Bb or F.
You could always write out parts for people to play so you don't need a rhythm section. Aebersolds are always good as well if you have access to a sound system.
I find the most enjoyable jazz clinics are ones you listen and play in, so try to incorporate as much interactivity as you can. High schoolers aren't the best people to sit and listen for an hour.