I would be questioning the appropriateness of sending a class full of kids to see a movie that is PG-rated. Our school keeps it to G-rated movies, in the older grades a permission slip is needed to watch a PG movie. My concern is that it seems like one of those movies that relies on violence to resolve the plot line (the first one did). That's questionable for a group of 5-6 year olds. How does that tie in to the curriculum? Surely there are milder movies they could attend, and if not maybe the teacher needs to find a broader range of excursion possibilities.

If that were me, I would be raising the issue if appropriateness of the excursion in the first instance. Then, it would be a choice of either accompanying him (if he felt ok with that) or keeping him at home for the afternoon.