Explain how the theme of bullying develops over the course of the story "All Summer in a Day."

From early in the story, we learn that the main character, Margot, is different from the other children. In our first introduction to her, she "stood apart from them, these children ..." We are then told a second time, "she stood alone." We also learn that she is "frail." It seems habitual to bully her, for the child William, when she doesn't answer his question about what she is looking at, tells her to "speak when you are spoken to" and shoves her. She doesn't respond or get upset, suggesting she is used to being treated this way. We then learn that she is an outsider who won't play with the other children in the tunnels of their underground home. We are told she is "different."  Finally, we learn that she remembers the sun, because while the rest of the children came to Venus at age two, she was older when she arrived. She is also adjusting so poorly to Venus that her parents are thinking of heading back to Earth. The other children resent her:

And so, the children hated her for all these reasons of big and little consequence. They hated her pale snow face, her waiting silence, her thinness, and her possible future.

We may be horrified, but we can't be too surprised at the biggest act of bullying in the story, when they lock her in the dark closet just as the sun is coming out.