I'm currently in my fifth year of directing Broadway Jr musicals, about to start Annie soon. Of all the ones in the Bway Jr collection, we've done Music Man, Guys and Dolls, Bugsy Malone, Once on This Island, and soon Annie. Of the past musicals, Bugsy Malone was the only one we found to not work with the age group. The Music Man was quite a bit of fun and I think nothing is funnier than middle schoolers putting on Guys and Dolls, and the boys in my group loved being gamblers. Once on this Island was a show no one in our group knew of, and it actually turned out to be a great show. Just beware, it's pretty much sung completely through with very few breaks for lines.
I would recommend The Music Man. It sounds like you have a decent sized group and it's a fun show that the kids will enjoy.
"I hope you're all happy, keeping Alan off the team. Couldn't you, for once, open your minds and hearts and hear something that he couldn't see? I, for one, am glad that Alan is blind, so he can't see what, I am so sad to say, you can't hear." -Jerri Blank