This could be a bit late, but Once Upon A Mattress could be a great school musical. We did it this past year and it's definately not a kid's musical if you have the regular version, there's also a school version but that's not as much fun (i.e. no pregnant, non-married women running around} It was a lot of fun but we had to cut a lot of the parts and still double up on roles since we had about 10 people in theatre this year. I myself had to play one role and then change while running across the back of the stage behind the backdrop to come out on the stage in time for the big musical number. I had roughly 12 seconds.