What happens to the narrator when he returns to school in A Separate Peace?
Gene returns to Devon and recalls his time there as a student. While walking the grounds, Gene finally comes to terms with his role in Finny's death, accepting his guilt but now finding some peace.
Up to this point in his life, Gene has been overwrought with his history with Finny.
He can't talk about Phineas because he can't accept the loss of his vitality, and he continues to feel guilty about his death.
On his visit to the school, Gene finds that things on the campus that once looked big now appear diminished. The jumping tree (where Finny was injured) now appears much smaller that it did in Gene's time at Devon.
We can read this diminution of scale as part of Gene's lesson. He returns to Devon to heal and also to put the past behind him. No longer is Gene dominated by the guilt he once felt for Finny's injury and death.
What he realizes in his return to the school is that his battle with that guilt is now over.
Gene had fought the real enemy during his years at Devon, the fear that comes from within and that causes individuals to build up damaging defenses that ultimately destroy them.