How has the atmosphere at the Devon School changed from Summer to Winter?

During the summer, the atmosphere at the school was easy-going, and the teachers were far more tolerant of the boys. In the winter, they clamped down.... rules were enforced, and the authorities took charge.

That was the way the Masters tended to treat us that summer. They seemed to be modifying their usual attitude of floating, chronic disapproval. During the winter most of them regarded anything unexpected in a student with suspicion, seeming to feel that anything we said or did was potentially illegal. Now on these clear June days in New Hampshire they appeared to uncoil, they seemed to believe that we were with them about half the time, and only spent the other half trying to make fools of them. A streak of tolerance was detectable; Finny decided that they were beginning to show commendable signs of maturity.

But all had been caught up, like the first fallen leaves, by a new and energetic wind.... this was [Devon's] one hundred and sixty-third Winter Session, and the forces reassembled for it scattered the easygoing summer spirit like so many fallen leaves.