The characters in A Separate Peace attend Devon boarding school, based on the highly elite Philips Exeter Academy in New Hampshire. These young male characters are the sons of wealthy and influential families. Social determinism comes into play because, having been given every advantage in life from birth, these young men are expected to become the future leaders of society: Supreme Court Justices, Senators, heads of banks and corporations. We recall, too, that the World War II period was the tail end of a time when social class was still firmly in place, and such opportunities as the GI Bill, which allowed returning vets a free college education, had not yet been implemented.
As a result of their upbringing, we can expect these young men to feel competitive with each other. While the competition in the novel is mostly presented as friendly, and the boys are primarily concerned with surviving the war in which they all expect to serve on graduation, competition rises up in an ugly way in Gene. Since he is the top student academically, and Finny is the top athlete, Gene suddenly develops the conviction that Finny is simply pretending to be his friend so he can sabotage his studies. Finny, Gene reasons, thinks he will become the top dog. This irrational sense of competition leads Gene to wobble the tree on purpose so that Finny falls and breaks his leg.
There is also a harsh, competitive edge to the way Brinker goes about setting up the mock "trial" that tries to prove Gene caused Finny's fall on purpose. These young men are all "friends," but darker, more destructive currents (which Knowles connects to the currents that started World War II) run beneath the surface of their relationships.