Saw this last night. I've never read the book but have to say I really enjoyed the film.
It was clear a lot of story had been either cut out or compacted, but despite this I thought it was intelligent and I liked the fact that it was quite brutal (compared to the sanitised lameness of something like the Hunger Games) and the actor playing Bonzo didn't bother me, I think it would have been cliche to have the bully be a much bigger guy, bullies come in all shapes and sizes.
I liked that it managed to show both sides of the argument and I thought the acting was of a high standard. Butterfield was maginificent, and that's the best I've seen Harrison Ford in a long time...kinda gives me (a new) hope for Star Wars VII
But if they'd called them buggers all the way through the film it would have just made me giggle...