Completely disagree that it's better to watch it at home. I think it's better to watch it with your peers so that you all have watched the same thing and can discuss it later.
I have a DS in Y9 and would have no problem at all if he had been shown SL. DH and I watched it in a big cinema when it came out. No one in the whole place ate their popcorn they'd bought or drank their Coke. You could have heard a pin drop throughout the whole film. I've seen it once. I don't need to see it again, but I do think it's a film that everyone should see at least once.
I think that, whereas when I was young we used to read about history, we didn't have all the distractions of XBox, PS, iPads, etc, or even late night/early morning TV. We were all used to reading quite a lot on the whole. I think using SL to teach history is no bad thing at all.
OP, YANBU to feel a bit upset for your DD, but YABVU to have complained to the school. Agree with the poster above talking about cottonwool kids. You cannot, and should not, protect your DC from everything.
I don't know, schools seem to be damned every way. Damned if they don't teach engaging lessons and damned if they do.