Au Revoir Les Enfants is a French film, which first premiered in 1987. At the time, there was an increased interest in France’s history with regard to World War II, which is why the film was very popular when it was first released. The film follows a boy, Julien, who attends a boarding school during the war.
In order to discuss the main theme of the film, you should point out that this film is unusual, as it is looking at historical events from a child’s perspective. This makes the main theme of the film very clear: the main theme, or social phenomenon as you call it in your question, is the loss of innocence.
A boy like Julien should be spending happy, innocent days with his friends during his time in boarding school. However, although his parents and the headmaster try to protect the Julien and the other students from the horrors of the outside world, Julien becomes more and more exposed to these in the course of the film. Instead of being able to lead the innocent life of a young boy, Julien experiences first-hand the atrocities of war, which will change his life forever. The fact that some of his fellow students are Jewish should not be a problem for a child—but it does affect Julien, as he witnesses how Jewish people are being discriminated against and how they are mistreated and arrested. An innocent child would have had the belief that good always wins over evil. This innocent view is being destroyed in the course of the film, as Julien has to learn that this is sadly not always the case.