The genesis of "Battle Royale" is quite fascinating. Wiki reports that the director, Kinji Fukasaku, was part of a class of kids that were enlisted in WWII to make munitions. Apparently the class was caught in artillery fire and had to hide under dead classmates. They then had to dispose of their fallen friends. This, combined with a new-found knowledge that their government lied about the reasons for WWII, caused a severe mistrust of adults, and inspired the film.
The essence of the film is that, at the turn of the century, Japan's economy collapses and employment skyrockets. Children become disillusioned with adults and start openly rebelling against teachers, police, parents, etc. The Government then creates and passes a law, commonly known as "BR." What the act provides for is that classes of 8th or 9th graders (I can't remember...) are taken to an island to fight to the death. And that's what happens in our film. It's "Lord of the Flies" meets "The Most Dangerous Game," meets "Pokemon."
A class boards a bus for a field trip, and after being drugged unconscious, arrives at an island. They realize that around their necks are metal collars with some sort of electronic device. They are greeted by a small group of armed infantry, and, to their unfortunate surprise, their teacher from the previous year. After reminding the class of how terrible they were to him, the teacher explains the situation. The kids are stuck on an island for the next three days. Only one kids will leave the island at the end of the contest, provided only that all the other kids are dead. If more than one kid is alive at the close of the contest, the collars will explode and kill all the kids. Any misdeed, tampering, etc., will cause the collar to explode. Each kid is issued a satchel with some provisions and a "weapon." I say "weapon" because the "weapon" each kid is issued is different. Some are issued a firearm, another a bow and arrow, the next a knife, and others were issued binoculars, and one kid got a gps unit.
Let the killing begin. There is a tender love story contained within the violence, but most of it is overshadowed with death and dismemberment. And it is very bloody. It's a slasher film at it's core, and I think it does an effective job at scaring the bejebbies out of you and providing realistic, yet stylized violence. Quentin Tarantino likes the film, to give you an idea. Very well done, very violent, very "not suitable for children."