Can you help me paraphrase the line "we could be thought of as a sign of the life the war was being fought to preserve."
To paraphrase text involves the restatement of a passage giving the meaning in another form. This usually involves expanding the original text so as to make it clear.
The main characters are 16 in the summer of 1942. They're still in boarding school, but many of the teachers have relaxed rules and standards. Gene (the narrator) suggets that this may be because the teenagers remind the adults that there may still be a chance for peace, a chance to return to a normal life. Thus the preceeding statement. Here's my recommendation for paraphrasing:
Our lives could be considered a representation of the existence and culture, which those fighting the war struggled to protect.
Although John Knowles's novel "A Separate Peace" was published in 1959, it is set in 1942. As its title suggests, the events and mood of the book are heavily influenced by World War II.
None of the boys seen in the book have actually been in the war, given that they are too young. Even so, the knowledge that the war is going on affects them and the American war effort affects their daily lives.
I would paraphrase the line you cite as:
The purpose of the war was to preserve the American way of life. You could say that we were an example of that way of life.