I would argue that William locks Margot in the closet because he is a bully and she is an outsider. Margot is very different to her classmates, having spent much of her life on Earth, while the others were all born and raised on Venus, knowing nothing but the Venusian rain. Margot has experience of the sun, which makes the other children, like William, jealous.

Margot is also an extremely shy, frail child, who “did not follow” when her classmates “tagged her and ran.” Feeling completely out of sorts away from Earth and everything she knows, Margot keeps to herself and simply doesn’t seem able to participate or fit in. While her classmates accuse her of being a liar, Margot genuinely remembers the sight and feeling of the sun. She misses it so desperately that her parents are considering taking her back to Earth. This is another factor which makes classmates like William jealous.

After watching at the window for a time, William becomes convinced that the sun is not going to appear that day and that all talk of it has been “a joke.” When Margot disagrees, William incites his classmates to help him lock Margot in a closet. This is how poor Margot comes to miss out on the hour of sunshine; when the beauty of the sun does appear, they all forget that they have imprisoned her.

"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.

William locks Margot in the closet because the sun is coming out and he does not want her to see it.

The problem between Margot and the other kids is that she is from Earth and they have lived on Venus for all of their lives.  They are jealous of Margot because she has seen the sun, and they do not remember the last time it came out when they were toddlers.  Margot also does not relate well to any of the kids from Venus.

The appearance of the sun brings out the worst in the kids. They are very excited because the sun only comes out once every seven years.

They turned on themselves, like a feverish wheel, all tumbling spokes. Margot stood alone. She was a very frail girl who looked as if she had been lost in the rain for years…

Margot has been on Venus for only five years. She remembers the sun, but the children cannot stand the idea.  They have turned to bullying to deal with the situation, because Margot has not assimilated into Venus life. She does not play their games and has not made friends.

"All a joke!" said the boy, and seized her roughly. "Hey, everyone, let’s put her in a closet before the teacher comes !"

The children do not really think about what they are doing when they put Margot in the closet.  They make it a “joke” even though it is supremely cruel.  When the sun comes out, the children simply forget about her in all of the excitement.  They seem to have intended to pretend to lock her in there so she would miss the sun, but then they actually did it.

This story is a good example of the pointless cruelty of bullying.  If someone is different, then that person becomes a target.  This is even more true with children, who are intolerant of uniqueness and privilege.