In "A Separate Peace", why is Brinker described as part of "winter session" effiency?

Brinker deals with the "business" of the school. He's involved in school politics and debate. He focuses on the in and outs of the school happenings. He sees it as his job to keep everyone is his place and focused on the correct goals. Brinker is the one who pushes Gene toward a decision about signing up for service. When Gene says this about Brinker, it is his reponse to coming off the carefree summer with Finny. Finny was about rule-breaking and enjoying being a boy. Even the summer classes that Gene takes are optional. When Finny is gone along with the summer, it's back to "business" with Brinker and the serious winter session. Gene falls under Brinker's serious spell, just like he fell under Finny's fun-loving one.