In A Separate Peace Leper says "only the people who've been evolving the right way survive"war. How does that apply to Leper, Gene, and Finny?
If you look at the characters of Leper, Gene and Finny, in the end, it is only Gene that actually makes it through the war with sound health and mind. Leper cracks under the pressure of training, and his mind becomes the victim to the rigorous schedule and lifestyle of a soldier. Finny, because he can't be on the front lines participating in the war, is torn apart by it and copes by making up stories about it. Also, Finny was so good-natured that Gene accurately concludes of him that he would be no good at fighting. He doesn't have the nature to be any good at war.
To "evolve the right way," you must steel yourself to the harsh realities of war; Leper was unable to do that, and Finny was caught off guard by the cruelty of war himself. In the book, Gene symbolically declares war on Finny through is declaration of it at the tree; his jealousy and insecurity led him to an act of thoughtless cruelty; in the end, that cost Finny his life. Finny was too nice to recognize that cruelty in Gene; when he finally was forced to confront it, he couldn't handle it, and it broke him. He didn't evolve with the proper amount of perspective and hardness to handle war. Leper did not evolve with the proper amount of hardness and resolve to survive it. Gene, however, did. He knew what cruelty was--he had seen it in his own heart. He knew the war was hard--he had seen it break Leper. He developed the attributes that needed to survive the war.
I hope that those thoughs helped a bit; good luck!