Thomas Wolfe wrote in his novel,
You can't go back home to a young man's dreams of glory and of fame,...back home to someone who can help you, save you, ease the burden for you, back...to the old forms and system of things which once seemed everlasting but which are changing all the time--back home to the escapes of Time and Memory
But Gene Forrester does "go home"; he does go back, but it is not to the "escape" of Time and Memory; Gene returns to Devon School in order to insert himself into Time and Memory so that he can analyze what it was that caused him to behave and think as he did as a student there. After he arrives, Gene looks for the tree whose limb he jousted, causing his friend to fall:
This was the tree, and it seemed to me standing there to resemble those men, the giants of your childhood, whom you encounter years later and find that they are not merely smaller in relation to your growth, but that they are absolutely smaller, shrunken by age....
By relieving his years at Devon School, his interactions with others, his private feelings, and the realization that his real enemy is something dark inside himself. Set against World War II, Gene's private war is not unlike the great war in Europe.
The narrator of A Separate Peace by John Knowles is Gene Forrester, and he tells the story from the perspective of time. In chapter one of the novel, he returns to the place where his life changed dramatically fifteen years earlier. He returns to the Devon School, an all-boys preparatory school in the hills of New Hampshire. Gene attended Devon in 1942, so of course the backdrop for the story is World War II.
Morse specifically, Gene returned to some of the places which hold the most memories for him from so long ago. He goes by some marble stairs in the First Academy Building, and they seem hard and unyielding. The second thing he goes to see is a tree on the bank of the river.
This was the tree, and it seemed to me standing there to resemble those men, the giants of your childhood, whom you encounter years later and find that they are not merely smaller in relation to your growth, but they are absolutely smaller, shrunken by age. In this double demotion the old giants have become pygmies while you were looking the other way.
We have probably all had the experience of going back to a meaningful place which loomed so large at the time but, years later, has shrunk both in size and significance. That is what Gene feels as he faces this tree and what it once represented.
Both of these places were prominent in Gene's past, and it is clear that he has spent fifteen years wondering what it would be like to come back here and see again the markers of his past. There is more to say about both of these places, but I do not want to ruin the story for you and will stop there.
The short answer to your question is that Gene, the narrator, comes back to the Devon School in New Hampshire after fifteen years. The things he sees now were there when he attended, but somehow they are different now. They have been changed by the passing of time.