Movies like Battle of the Classes

As everyone knows, children don't care about differences between social classes, skin colors, or religions. But then why does Corentin, Paul and Sofia's 9-year-old son, only have friends like himself at Bagnolet's school? And when his friends all leave for a private school in Paris, his parents are frightened. From now on, Corentin is the only one in his class. But the only what?