The climax occurs when John and Lorraine confront Mr. Pignati about the truth of their lies and the damage they've inflicted on him. Specifically, it's the realization that they've deliberately and cruelly manipulated him, even after he's opened his heart and home to them. This revelation leads to Mr. Pignati's heartbreaking emotional collapse and his subsequent death, directly resulting from the stress and disillusionment caused by the boys' actions.
While the party and the subsequent destruction of the house are significant events, they are *a result* of the climax. The boys' deception and the profound emotional damage they inflict upon Mr. Pignati is the true turning point—the moment of highest tension and irreversible consequence that defines the story's core conflict and leads directly to its tragic conclusion.