The boys change in chapter 30 by becoming more physically aggressive toward Stanley and excluding him from their group.
Stanley is a perceptive character, and upon arriving at Camp Green Lake, he proves strategic in making alliances that will improve his chances of survival. Thus, he wins X-Ray's favor and reaches out to Zero, helping him learn to read. Stanley is fairly successful in blending in to the established culture of the boys and finds a degree of acceptance.
This shifts in chapter 30 as the boys grow increasingly resentful of Stanley's agreement with Zero. Zigzag accuses Stanley of working Zero like a "slave" and insists that their arrangement is "a good deal" for Stanley, allowing him to avoid working as hard as everyone else. Stanley hopes that after lunch, the boys will "leave him alone" when he returns to doing the digging himself.
Instead, Zigzag and Squid approach Stanley during lunch, trying to provoke him further. Stanley tries to avoid conflict, but Zigzag pushes him. Mr. Pendanski encourages Stanley to "teach [Zigzag] a lesson," and Stanley makes a halfhearted attempt to defend himself. In response, Zigzag pummels Stanley until Mr. Pendanski intervenes.
The boys then snitch on Stanley, telling the Warden that Stanley isn't doing his "fair" share of the work. Their behavior conveys a shift in their treatment of Stanley, signifying that he does not belong, which furthers themes of alienation.