Virtue is having moral excellence or conforming to a standard right. This is an interesting book through which to assess "true virtue" because of the era and the age of the characters.
In this book, there is a great wrong committed against the main character Finny. Gene has committed this crime and does indeed want to correct it. But when upon trying to confess, Gene continues to allow Finny to think that it's just a joke and the truth is never exposed until much later.
That exposure is virtuous. Trying to get to the bottom of something so that people can move on and not be held back by a corrupt act is important. However, those seeking the truth might have done so in effort to hurt or destroy. Brinker's motivation is difficult to discern because he could have done it out of jealousy of Gene's friendship and loyalty to Finny.
True virtue would be doing the right thing. Most of this book demonstrates characters struggling with the decision to do the right thing but often making the wrong choice.
In the book A Separate Peace true virtue appears to represent the idea of being a good person and a true friend with nothing: expected in return except friendship. The Encarta World Dictionary lists virtue as "the quality of being morally good or righteous."
In the book Gene experiences jealousy over Finny who is popular and outgoing. Finny is able to charm the school professors while getting away with mischief. Gene is envious of Finny. Finny is unaware of the envy and considers Gene to be his best friend. He is always trying to get him involved in his activities.
Gene misinterprets Finny's actions that cause Gene to miss some studying time as Finny competing with him over grades. He becomes angry at him because he believes he is trying to sabotage his studies. As a result Gene joggles a tree branch causing Finny to be seriously injured.
The idea of true virtue is central to the novel. Gene learns that Finny had never wanted anything but his happiness and had never tried to sabotage him academically. By looking closer at Finny's noble and honest personality, one learns that he is indeed very virtuous and has become a victim of his own virtue.
Gene, on the other hand, tries to do the right thing and tell Finny what he ahd done. Finny can not hear it emotionally though. Gene is left holiing his own guilt and wondering if he has any virtue.