Don't knock it, the ending is a CrowningMomentOfHeartwarming, but what bugs me is the FridgeLogic that someone (probably his wife or son), went through Mr. Holland's private papers to find the Opus he had written, photocopied it and had the entire band practice it ''without Mr. Holland's permission or knowledge''. Then they expect him to conduct it cold turkey in front of the auditorium.
** Yes, that's a leap of faith that the viewer has to make. It helps, I suppose, that the piece is not really a symphony in any traditional sense. It's set to a rock beat and is fairly predictable in its chordal structure (well, it's really the theme music of the film, isn't it?). Had it been a demanding piece with tempo and time changes, the premise would be much harder to accept.
And, yes, the other issue is the secrecy with which the whole thing must have been achieved--rehearsals at times and in places where Holland would not have been expected. They could hardly have rehearsed at the school--he seemed to know everything that was going on at the school at the end of his career.