"All Summer In a Day" is a story about cruelty. In this story, Margot is locked in a closet while the rest of the children are able to enjoy a brief respite from the otherwise incessant downpour that marks Venus's climate.
When describing the story's depiction of bullying, I think there are several factors that should be accounted for. First of all, this story does seem to reflect a theme of crowd mentality: note that it is not just William who bullies Margot; rather, the entire class is complicit in locking Margot in the closet. As Bradbury writes,
They surged around her, caught her up and bore her, protesting, and then pleading, and then crying, back into a tunnel, a room, a closet, where they slammed and locked the door. They stood looking at the door and saw it tremble from her beating and throwing herself against it.
Furthermore, keeping in mind this theme of crowd mentality, it might be useful to think about the story's ending, where the children return to the closet, feeling a deep sense of shame and guilt over what they had done. Consider the ways in which people will often act differently when part of a large enough group, easily swayed to adhere to the larger swings of emotions of that group. I would suggest that the same psychological reality is in play here, in this particular Bradbury story.
Moreover, Margot herself is defined as an outsider among the children: she was born on Earth rather than Venus and has difficulty assimilating into the life the other children take for granted. There does seem to be an element of jealousy at play, given Margot's memories and knowledge of the sun—memories and knowledge the other children do not possess, given that they have lived on Venus all their lives. These factors are critical in shaping why the other children bully and torment Margot to begin with.