It's Great to be Young!.

IT’S GREAT TO A YOUNG stars John Mills as Dingle an easygoing high school teacher. When autocratic new headmaster Frome (Cecil Parker) begins imposing all sorts of repressive rules, Dingle does his best to stand up for his students, only to be dismissed for his troubles. The kids conspire to not only reinstate their favourite teacher, but to circumvent Frome's refusal to purchase new instruments for an upcoming music festival.