Three factors that explain the change of mood between the summer and fall sessions are as follows:

First, Gene describes himself and his Devon summer school friends as in a liminal space of "shaky transition" from "lower Middlers" to "upper Middlers" in the school hierarchy. When they return to school in the fall, they will have a definite status and place in the order of the prep school.

Second, the relaxed and half empty atmosphere of the prep school in summer allows for some breakdown in the hierarchy between teachers ("Masters") and students. This is helped by summer substitute teachers, such as Mr. Prud’homme. As Gene remembers:

Mr. Prud’homme released his breath with a sort of amazed laugh, stared at Finny for a while, and that was all there was to it.

This was the way the Masters tended to treat us that summer. They seemed to be modifying their usual attitude of floating, chronic disapproval. During the winter most of them regarded anything unexpected in a student with suspicion, seeming to feel that anything we said or did was potentially illegal.

Third, the summer session represents peace to both the students and the teachers. This is in the midst of the World War II years, and in the fall, the reality of the war hovering on the horizon will be a distinct drumbeat. For now, however, it can sink away as if it does not exist.

The summer session at Devon embodies the last days of freedom the boys have before the War hits home in a hard way.  During the summer, the teachers are intentionally lax on the discipline of the boys, and the boys enjoy extra free time around their classes.  Meetings of the Summer Suicide Society, games of Blitzball, and trips to the beach all demonstrate this free spiritedness.  Finny's ability to get out of any trouble also enables this attitude.

During the fall, structure and discipline return to Devon along with the rest of the student body.  The chapel services are more serious, the rules are enforced strictly, and wartime activities take up free time.  The boys shovel snow on the tracks and talk about enlisting; Leper runs off to enlist with the ski patrol.  Also, Finny's absence takes away some of the carefree attitude of the summer.  When he returns, his injury continues to dampen the spirits of the boys.  Fall is the time when they begin to grow up and to face the real world that they spent the summer avoiding.