A big thanks to everyone who replied. I read them all, and wanted to reply to each of you!

The school (there are 130 preps) is studying fairy tales, culminating in watching a fairy tale. I feel it is a terrible choice of movie, but want to resist the urge to be 'that parent' who always complains. Let's face it, deep down they want to take the kids on an excursion (yes, their first prep excursion) and this just fits the timetable.

I have discussed with his teacher and I can go with him. They will be discussing the movie in class formally (as well as in the playground, I'm sure). I have opted to wait a day or two before I discuss it again with him.

My son has never had a day off school. We has sent home once at 2pm because he fainted in class. He's thriving at school educationally and has never asked to not go.

I trust him with his feelings. I have to trust him about this. Sure, he's six, but that doesn't, to me, invalidate his wishes. I do not believe forcing him to attend this movie will be educational. Nor do I think it will benefit him emotionally or help him deal with his anxiety over it.
He doesn't really like swimming. But by god, every week I make him go. And he tells me what he thinks of it, and me. But he still goes.

He and I have been to two movies. The first time was The Lorax. His choice. He was terrified. He hated it. Spent the whole time on my lap under my arm with my cardigan over his head and his hands over his ears.
The second was Turbo. He spent 50% of the time on my lap with his hands over his ears screaming la la la to keep the noise out. The other 50% of the time he was on the floor with his head buried in my lap.

Coincidentally about 3 months ago my 4yo picked HTTYD from the video store. The 6year old hid in his room. Cried and sobbed when they put toothless 'in jail' because he knew toothless was innocent and didn't want him hurt.

I'm not sure that "emotional challenges" is an appropriate word to describe a trip to the movies. Maybe for some children, but not mine. Trauma. That's a better word. Like surprised said, she can laugh about it now, but why as adults we get to choose whether to subject ourselves to it, but children somehow get told it will be a good challenge for them? "It'll help him learn to persist when things get tough"? No, it'll remind him that he's different and that he needs to suck up his fear and emotions and not show them to anyone.

If it was a rock climbing excursion, yes, he'd be going. In fact, I know damn well he wouldn't ask not to, even though he probably wouldn't want to. In fact, it's dress up day soon and he hates dressing up, but has never once mentioned not going.

(I have since learned that a complex about an hour away does sensory movie sessions sometimes. I might try that sometime with him. But no hurry).

Thanks for the replies. It's nice to know I am not the only one who a) thinks the movie is a terrible idea for any child and b) thinks my little guy has a right to have his fears acknowledged and a right to say yes or no to things that happen to him.