Jack Black and Joan Cusack. They're fantastic in those roles in the movie and, if for some reason "School of Rock" was made into a musical, I would want them to remain in those roles. Dewy Finn is not Dewy Finn if he isn't played by Jack Black.
Forget the fact that no one else could play Jack Black's role. The show would be impossible to do because you absolutely need real kids who really play the instruments and sing. it would be a casting nightmare.
It just doesn't seem like it would make a very good musical. I agree with Jon, to find kids up to par with the kids from the movie, they would have to use, well, the kids from the movie, but if you're just going to use the cast from the movie, what point is there to turn it into a musical when the movie is already so great? I've seen this movie more times that I would like to admit, and I don't see it working as a musical.
Think of the nightmare of keeping it cast. Shows like The Sound of Music have so much trouble keeping those 7 kids looking the right ages, and if you added musicianship on top of that, no way. I love the idea, but I don't think it's very feasible.
I think Jack Black is actually quite a talented musician. I wouldn't mind seeing him on Broadway.
"Writing should be easy, like a monkey driving a speedboat..." -[title of show]
"I'm tired of following my dreams. I'm just going to ask them where they're going and meet up with them later." -Mitch Hedberg