Understanding symbolism in a Separate Peace will give you a greater understanding of the novel.
A Separate Peace Symbols
The Summer Session at Devon: Rules are relaxed. The weather is nice. Students like Finny excel. It is symbolic of childhood innocence.The summer session symbolically ends with Finny's fall from the Suicide Tree.
The Winter Session: Rules are enforced. The weather is depressing. Students like Brinker Hadley excel. The winter symbolizes the end of childhood and innocence and the encroachment of the war.
Finny's Fall from the Suicide Tree: Finny's fall is a fall from innocence. It represents evil being unleashed. The idyllic summer session comes to a close, ushering in hardness and cold. Taking the concept of the fall to the next level, Devon during the summer session can be considered Edenic; Gene is the serpent, and Finny the forbidden fruit, plucked from the tree. The winter session at Devon, therefore, becomes the harsh, cruel world.
The Light Surrounding Finny and Gene on the Suicide Tree: Leper is called as a witness at Gene's mock trial. He testifies, "I could see both of them clearly enough because the sun was blazing clearly around them...The two of them looked as black as--as black as death standing up there with fire burning all around them" (174). The fire surrounding them is symbolic of being put through the refiner's fire, of being tested. Gene fails the test.
World War II - World War II represents the imminent trials of adulthood and the triumph of evil over good. Not even Devon with its austere architecture (think Edgar Allan Poe's Masque of the Red Death) can keep out the war. The only thing that causes Gene to forget the war is Finny, who represents innocence and youth. It is not coincidental that after Finny's death, the paratroopers take up residence at the school; nor is it coincidental that before Finny returns for the winter session the boys engage in war duties, but when Finny returns, no such opportunities arise.
Leper - Those looking for biblical symbols need look no further than the school outcast. Much like Lepers in the Old Testament, who were removed from society, Leper too is an outcast at Devon, and later in the army.
The Suicide Tree - The novel's most prominent symbol is the tree from which Finny falls. It represents the fall from innocence and the transformation from youth to adulthood, a transformation Gene makes, but Finny--similar in this respect to Holden Caufield in The Catcher in the Rye--does not.