What quote from A Separate Peace shows that Brinker Hadley is smart?

Brinker Hadley is Finny's foil and one of the conservative leaders at Devon. In chapter 6, the reader is introduced to Brinker Hadley, who moves into the same room Leper Lepellier stayed in during the exhilarating summer session. Gene pauses and thinks before entering Brinker's room and comments that he is not yet prepared to see Leper's snail collection replaced by Brinker's neat files. Gene proceeds to describe Brinker as Devon's "dominant student" and says that he would normally be attracted to the idea of visiting Brinker's room if it were not for the "gypsy summer," which has had an enormous effect of his character. Gene then mentions,

Now Brinker, with his steady wit and ceaseless plans, Brinker had nothing to offer in place of Leper’s dust motes and creeping ivy and snails.

The phrases "steady wit" and "ceaseless plans" are used to describe Brinker, which indicate that he is an intellectual character. As the novel progresses, Brinker establishes himself as a serious, rule-following student who spends a significant amount of time studying and encouraging Gene to enlist. Brinker Hadley is considered a foil to the spontaneous, rule-bending Finny, whose charisma and carelessness are his defining features.