The sentence that shows how the setting impacts the plot is: "On the appointed day, I ventured into the hallway outside the teachers' lounge so I could ambush Mr. Grayson with questions the guys provided me."
This sentence establishes a specific location (the hallway outside the teachers' lounge) that directly influences the events of the story, as it is the setting where the main character's actions (ambushing Mr. Grayson) take place, setting off the chain of events leading to the surprise party.