The kids will sell this show no matter what. They were the best part of the movie.
To me, though, Joan Cusack was the film's secret weapon - it's a tour de force performance that should've garnered Cusack a Golden Globe nomination at least; she's hysterical in that role. Beth Leavel in the musical, anyone?
The thing that makes School of Rock work, and which separates it from being the Music Man reboot it was intended to be, is that Dewey may be a shady character, but he is not Harold Hill.
Harold pretends to be a teacher to trick people into giving him what he wants. When his encouragement actually pays off and the students take to music tentatively, he is surprised. Dewey, on the other hand, may be a shyster, but he genuinely believes he can use his limited talent but boundless enthusiasm to make something great out of the limited enthusiasm but boundless talent of these private school kids.