One possible thesis statement could suggest that A Separate Peace is a story about many different wars - the war Gene fights within himself, the war the boys fight against the world in and outside Devon School, the war between the nations and generations.
A metaphor used to describe Gene's war with himself could be "My brain exploded. He minded, despised the possibility that I might be the head of the school." Gene's brain did not literally explode, but the thoughts that he was having seemed explosive. A simile from that same paragraph continues the thought process: "up like a detonation went the idea of any best friend."
The Winter Carnival could illustrate the war against influences from the outside and the boys' fight against those events. The Carnival represented rebellion against all authority, starting with that of the school itself. The attitude of the boys is metaphorically compared with a horse race.
twenty boys, tightly reined in all winter, stood now as though with the bit firmly clamped between their teeth, ready to stampede.
The hard cider, a symbol of rebellion against rules and authority, gives rise to one possible simile. "The carnival's breaking apart into a riot hung like a bomb between us." There was no bomb, but there was Gene's act of forcibly pouring the cider down Brinker's throat.
And the Carnival was a symbolic victory in the battle against World War II, ever looming in the background but successfully escaped on that afternoon.
it was this liberation we had torn from the gray encroachments of 1943, the escape we had concocted, this afternoon of momentary, illusory, special and separate peace.
As the war physically arrived at Devon in the form of troops and trucks and sewing machines, the war against the War is reflected in the contrast between the school and the outside.
Peace lay on Devon like a blessing, the summer's peace, the reprieve, New Hampshire's response to all the cogitation and deadness of winter.
The war between Brinker and his father metaphorically echoes the war between the generations of soldiers fighting in the war of the past and the war of the present.