Battle Royale book is similar to The Hunger Games book, but Battle Royale is better imo. I really liked the movie despite cringing at some parts (like the crotch knife scene). It does play up the gore, but you really can't do a story like that properly without the violence. The only thing that really bugged me about the movie was the fact that many of the actors didn't look like they were middle school students.
To be fair with stuff set in schools etc, it's inevitable to have actors a good few years older than the parts they're playing (blame child labour laws - Will Ferrell in Zoolander was right!).