In the fourth chapter of The Chocolate War, Archie is called in to Brother Leon's office. Brother Leon wants to meet Archie to talk over the upcoming cholocate sale, which is Leon's pet project and important to the school.
Archie is surprised by two things in this meeting. First he realizes that Brother Leon is sweating and nervous. When Archie sees this a bit of insight falls into place for him regarding Brother Leon and most adults - "they were vulnerable, running scared, open to invasion."
This insight gives Archie a certain power in the meeting and allows him to bait Brother Leon and play with his emotions.
Later in the meeting, Archie is surprised to realize that Brother Leon is willing to acknowledge the existence of the Vigils and Archie's role with them. Though Leon does not say the same outright, he clearly acknowledges the secret group.