Concrete

Concrete also released as Schoolgirl in Cement is a 2004 independently produced Japanese film directed by Hiromu Nakamura. The film is based on the case of the murder of Junko Furuta and it deals as much with the social factors that produced Furuta's four assailants as it does with Furuta's suffering at their hands.

Synopsis from Wikipedia, CC BY-SA 4.0. Everything below written here.

Poster © the film's rights holders.