In A Separate Peace by John Knowles, Gene says "Peace had deserted Devon." What does this hint at as a possible theme of this book?

In A Separate Peace by John Knowles, Gene says "Peace had deserted Devon" at the beginning of chapter 6. He is referring to the fact that the summer session has ended, and everyone is back to school for the start of the fall. It also means that he is starting his senior year, which is the last year of school he will attend before enlisting in the army. Therefore, a possible suggestion for a theme might be that nothing can ever stay the same because the peaceful summer ends and war awaits each boy at the end of the year. Finishing high school and entering the war might also be symbolic of growing up and accepting adulthood.
First, take into account the fact that the summer session involves relaxed classes and teachers; so for the boys, they experience more peace overall in the summer. The winter session, however, includes more students attending and returning to strict adherence to the school's rules. For example, when Phineas and Gene miss dinner because they are horsing around and wrestling, the teachers don't penalize them in the summer. If such a thing were to happen in the winter session, the boys would face demerits or punishment. Gene describes the differences between the Summer and Winter Sessions as follows:

"The Summer Session—a few dozen boys being force-fed education, a stopgap while most of the masters were away and most of the traditions stored against sultriness—was over. It had been the school's first, but this was its one hundred and sixty-third Winter Session, and the forces reassembled for it scattered the easy going summer spirit like so many fallen leaves" (72).

The "forces" reassembling for the fall means that not only does the official headmaster return for the beginning of a new session, but other teachers and the rest of the student body come back, too. In addition to school starting, World War II looms in the distance for the seniors, and it will end their childhood peace. This causes the boys to feel anxious for the last year of school. Another possible theme might be that war is the only thing they have to look forward to so they better live life the best way they know how before it happens. Therefore, the above passage shows Gene realizing that the relaxing and peaceful days of summer are gone and have been replaced with hard work, a lot of study time, and the eventual enlistment awaiting the seniors at the end of the year. "Peace had deserted Devon," because the summer is over, and after graduation, the boys go to war.