Normally I say that if a school does something, it is important so sadly, no refusals unless it can kill (eg severe nut allergy child entering squirrel enclosure).

But seeing a film, one that is scary for under-7s? Um, what is the prep thinking of? If Liebling's school were to do this, I would be cross. It is not necessary and tbh, even with a "happy" film, the cinema experience can be overwhelming, loud and scary. Even for adults!