When the seniors begin to have a snowball fight, they ask Gene to join in, but he doesn't really want to (pretending that he has something else that he has to do). The boys keep at him and finally he gives in and joins them. The symbolism, here, is that this snowball fight mirrors the upcoming trial scene. There is a betrayal that happens in the snowball fight, and it foreshadows the betrayal that will come out in the mock trial. In both cases, no side really wins. And this is an even greater statement about war in general.