How do Leper's theories about skiing set him apart from the other boys in A Separate Peace?
Leper is not caught up in the ideas of patriotism and duty like the other boys. When a large snowstorm hits the area, the rest of the boys from Devon volunteer to clear a rail yard. There is a sense among the boys that this is the right thing to do as a lead up to military service.
The exception is Leper Lepellier, who, seemingly oblivious to what is going on around him, goes out skiing, looking for a beaver dam.
This episode is one example of Leper's social distance from the other boys. It is also an example of his lack of practicality. Leper, as we see him here, is a romantic. In this way he is not unlike Finny, which is perhaps why Gene can appreciate Leper's choice to ski instead of volunteering with the other boys.
Gene alone appreciates Leper's innocent devotion to the natural world; the other boys make fun of him, regarding him as odd and completely out of touch.
Not long after this episode, Leper's romantic nature leads him to enlist after watching an Army film that appeals to Leper's imagination. Failing to realize that the film does not depict military life as it really is, Leper joins the Army and quickly goes AWOL, unable to cope.
No ideas of duty or patriotism can convince Leper to return. His imagination has been betrayed and he has learned hard lessons about himself and his limits in his short time in the armed service.
Unlike the other boys, Leper cannot adapt. He is not a "joiner". Too innocent, too romantic, or simply too dreamy, Leper attains a separate peace of his own, of a sort, long before Gene realizes that this type of choice is possible (to serve or not; to be a certain kind of person or not, etc.).