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.