"We'd better hurry or we'll be late for dinner," I said, breaking into what Finny called my "West Point stride." Phineas didn't really dislike West Point in particular or authority in general, but just considered authority the necessary evil against which happiness was achieved by reaction, the backboard which returned all the insults he threw at it (1.46).
Right away, Gene paints a portrait of Finny that pits him against the authority that will later take over in the Winter Session at Devon. But the fight is a friendly, sporting one – that's how Finny competes.
The Devon faculty had never before experienced a student who combined a calm ignorance of the rules with a winning urge to be good, who seemed to love the school truly and deeply, and never more than when he was breaking the regulations […]. The faculty threw up its hands over Phineas, and so loosened its grip on all of us (2.8).
When Phineas leaves Devon, then, it follows that the masters can again tighten their grip. Which they do.
I noticed something about Finny's own mind, which was such an opposite from mine. It wasn't completely unleashed after all. I noticed that he did abide by certain rules, which he seemed to cast in the form of Commandments. "Never say you are five feet nine when you are five feet eight and a half" was the first one I encountered (3.6).
What is it that governs Finny's set of rules? Is there any sense of morality or logic to his system?
We met every night, because Finny's life was ruled by inspiration and anarchy, and so he prized a set of rules. His own, not those imposed on him by other people, such as the faculty of the Devon school. […] We met every night. Nothing could be more regular than that. To meet once a week seemed to him much less regular, entirely too haphazard, bordering on carelessness (3.4).
Finny's own "set of rules" comes to increasingly govern Gene's outlook and behavior as the novel continues. Even after Finny's death, Gene continues to live by these principles.
"You aren't going to start playing by the rules, are you?"
I grinned at him. "Oh no, I wouldn't do that," and that was the most false thing of all (5.82-3).
Following Finny's rebellious lead is one of the ways that Gene shows loyalty to his friend. Without Phineas, he has no reason to transgress.
Still it had come to an end, in the last long rays of daylight at the tree, when Phineas fell. It was forced on me as I sat chilled through the chapel service, that this probably vindicated the rules of Devon after all, wintery Devon. If you broke the rules, then they broke you (6.7).
This sounds like more justification on Gene's part. It wasn't the rules that broke Phineas; it was Gene.
Brinker the lawgiver had turned rebel for the duration (9.39).
This is the last straw. Phineas's return to Devon has turned the tables and reversed the transition from summer to winter, peace to war, anarchy to order.
"If a war can drive somebody crazy, then it's real all right. Oh I guess I always knew, but I didn't have to admit it" (11.84).
Here we see another example of Finny's rules. His fantasy world, in which there was no war or enmity, turned out to be precariously balanced on his delicate system of principles.
"What'd you come around here for last night?"
"I don't know. […] I had to. […] I thought I belonged here" (12.41-2).
At Devon, there are rules to govern friendship, too, though they shift as Gene's relationship with Finny evolves.
My brief animosity, lasting only a second, a part of a second, something which came before I could recognize it and was gone before I knew it had possessed me, what was that in the midst of this holocaust? (12.34).
As change comes to Devon, even the rules are affected, or so Gene would like to think. Crimes become relative, guilt is redefined.