In "A Separate Peace", what details does the author use to show the ways in which the war is changing the atmosphere at Devon?

There are details in the very first chapter that show how things have changed at the school.  There had never been a summer session before the war, and Gene, Finney, and the other students on their way to being Upper Middlers are the first group to have a summer session.  Gene refers to how the seniors are "caught up in accelerated courses and first-aid programs and a physical hardening regimen..." (15) 

In Chapter 2, Gene makes reference to how the masters treat the students over this summer, saying "a streak of tolerance was detectable" (23).  Gene speculates that the students reminded the masters "of what peace was like...a sign of life the life that the war was being fought to preserve" (24). 

As the novel continues, there are other examples of how the war affects the school.  Gene reports that "day after day we were exhorted to new levels of self-deprivation and hard work, with the war as their justification" (118).  Conversation among the students is about who will enlist, and who will not.  The spector of war is a constant theme at Devon, for students and faculty.