What is the psychological and thematic significance of the "secret guards" and the fantasy of the plot against Miss Geraldine?

Kathy’s narrative goes further back, to her earliest days at Hailsham. She becomes friends with Ruth around age 7, and they often play games involving pretending. One day, Ruth invites Kathy to join the “secret guard,” a group of girls who pretend to be bodyguards to Miss Geraldine, their favorite guardian. They imagine a plot to kidnap Miss Geraldine and take her into the terrifying woods that surround Hailsham.

Ruth remains aloof in this section but begins to recognize that social relationships involve give-and-take; she fervently searches for a way to repay Kathy after the latter refrains from humiliating her about the pencil case. This contrasts sharply with the way she handled the chess incident several years earlier; rather than being grateful to Kathy for keeping her secret, Ruth expelled her from the secret guard.