What effect might the war have on the "Upper Middlers" (the juniors) like Gene and Finny?

The seniors were being trained for the war, and many of the boys had already been sent off as soldiers. For Gene, Finny, and the rest of the "Upper Middlers," the absence of the seniors put pressure on them to grow up and take their places. The boys are too young for the draft, but too old not to understand that how the war affects their former school mates or what might possibly be in their own futures.