The movie is based in a Japan where high unemployment had driven up the crime rate, especially amongst the young to the point where society turned against juveniles. That led to the Millennium Educational Reform Act where a single class of students was placed upon an abandoned island where they would have to fight each other to the death.

https://www.amazon.com/Battle-English-Subtitled-Tatsuya-Fujiwara/dp/B0079NQZLE