The title A Separate Peace refers to the peace reserved for schools that are separated from the war that goes on outside of the campus. The environment that the school provides is an environment where the war is non-existent, where the hatred and the violence that the world had to offer did not sink in for the boys attending Devon high school. In fact for Finny, the war didn’t even exist, it was just a made up thing for him. The school of Devon was a separate piece, isolated from the outside world.  Although they can be prepared for the war with drills and things of that nature, they can never truly teach to the kids at Devon to abandon their peaceful nature as just teenage boys. The hate the world has is something completely unknown to the boys. The violence was unknown to them. It was almost as if they were imprisoned for their own benefit.

It also could be a reference to Gene reserving a separate piece of body to be Finny, to live through Finny. It was his duty to Finny to be Finny. It was only through living through Finny could Gene achieve peace for injuring and inevitably killing him.