My ds2 is a sensitive little flower; I wouldn't take him to Azkaban at the cinema for instance, or any of the LotR. But the reviews I've read for Narnia say it's not so scary, and I will take him to that.
However, I think your concerns about taking a whole class of children with fewer adults are absolutely spot on. I would not let ds2 go and see this film without one of us present.
(Have you read the book/know the plot? It's pretty heart-rending in places. I know ds2 will cry and want to sit on my knee.)