Why do the boys invent stories of Leper as a war hero?

Led by Brinker, they mockingly envision Leper as a war hero. Gene himself notes that by talking and laughing about Leper's heroics, he and his classmates are able to personalize the war.

"In a rather remarkable way, Leper becomes more important and is taken more seriously by the boys once he has left and gone to war. He is a symbol of the heroism and interesting deeds of war, a representation of all the brave and wonderful successes of the American troops. Their support of "Leper" is another way to show patriotism and feel involved in the war, without having to face the harsh realities involved in the real events."

I think the "war hero" thing was more or less done mockingly. Leper’s decision affords the reader several insights into his classmates, as the boys react in telling ways. First, they respond with disbelief, and because they find the idea of Leper in the army so unimaginable, the war becomes to them more distant and alien than ever. Later, however, when they do begin to consider Leper’s enlistment as a possibility, they turn the issue into a joke. Led by Brinker, they mockingly envision Leper as a war hero. Gene himself notes that by talking and laughing about Leper’s heroics, he and his classmates are able to personalize the war. When they imagine one of their peers involved in grand historical events, the war suddenly seems more on their level, less intimidating; after all, if Leper can be a hero, then anyone can. Thus, the boys’ anxieties about wartime failure, about being “the Sad Sack, the outcast or the coward,” can be set aside.