How did the Devon school change once the fall semester began?

According to Gene, the new session "scattered the easygoing summer spirit like so many leaves"; the simile reinforces the shock of the rigorous, crowded fall session, after the ideal languor of the long summer. In the hustle and the renewed conservatism and law-enforcing of the fall session, Finny and Gene's glorious summer already seems like a thing of the past. The contrast between the summer and the fall, like the contrast between the winter when Gene revisits the school and the summer he describes, reinforces the rarity of the days they had, and reiterates another theme, of how fleeting the past, and the best days, can be. The change of seasons also foreshadows a change in Gene's life, and in his and Finny's relationship; with the passing of time, they will not be able to regain what they had in their ideal summer together, and their relationship when Finny comes back will most definitely be changed.