While the river is an intended symbol of the division between the carefree youth and the war, and the tree, also seems intended to symbolize the breaking of friendship between Finny and Gene, in Chapter 1 of A Separate Peace Gene Forrester as narrator describes the First Academy Building that has a Latin inscription over the doorway that reads "Here Boys Come to Be Made Men," this doorway with the large cupola and a bell and a clock and its inscription could be also be a symbol, and, indeed, one that is representative of the novel.
For, once Gene and Phineas, Brinker, Lepelier, and the others pass through the doorway of the First Academy Building, they can never go back to what they were before. And, it is after passing through the door and answering many bells and spending much time (the clock) at Devon that events happen to unalterably change their lives. For Leper, the War alters his life irrevocably as the sensitive young man cannot cope with the rigors of the Army into which he has enlisted. For Phineas and Gene, it is the war in Gene's heart, the "something ignorant in the human heart" that is the life-changing element as he imagines that Finny is jealous of his academic ability and tries to make him fail. Gene envies Finny his easy way, his joie de vivre [delight in life] and his absolute lack of any petty emotion, contriving out the "ignorance in his heart" a rivalry between him and Finny. It is only Finny who has the changes in his person imposed upon him as his heart does not acquire envy or fear. But, he does come to know that the "separate peace" he and Gene experience on the afternoon of the Winter Carnival is special, but illusionary because Gene has constructed an imaginary Maginot Line of defense between them, one that leads to Finny's broken leg. Certainly, it is a different Gene Forrester who returns to Devon School and walks again through the doorway of the First Academic Building in order to understand what occurred within him at Devon School, a doorway through which he must walk again in order to truly understand who he is, yet, always, one through which his life has been irrevocably altered.
the symbol can be a peace sign