Why do the other children bully Margot in "All Summer in a Day"?

Ray Bradbury's short story "All Summer in a Day" is set on the planet Venus, where it rains constantly and the sun only comes out for a few hours once every seven years. The characters are a class of nine-year-old children, most of whom

had been on Venus all their lives, and they had been only two years old when last the sun came out and had long since forgotten the color and heat of it and the way it really was.

Margot is one of the children in this class, but she is different from the others, an outsider to her peer group. She's a shy girl, "very frail," like "an old photograph . . . whitened away." She is also so quiet that "if she spoke at all her voice would be a ghost." When the class bully, William, shoves her and shouts at her, she stands there passively, neither resisting nor answering him. She "[plays] no games" with the other children; even if they try to include her, "she [stands] blinking after them and [does] not follow."
All of Margot's traits—her frailty, her timidity, her silence, her passivity—are enough to make her a target for bullying by the other children, but

the biggest crime of all was that she had come here only five years ago from Earth, and she remembered the sun and the way the sun was and the sky was when she was four in Ohio.

The combination of Margot's meek, awkward personality and her enviable history as a child from Earth is more than the other children can bear. It seems deeply unfair to them that someone like Margot can remember the sun and the sky and a world without endless rain, when she is so weak and unpopular. Whenever Margot dares to say anything about the sun, the children shout "You’re lying, you don’t remember!"
On the day the story takes place, the sun is due to emerge from behind the clouds for its brief once-every-seven-years appearance. The children are all excited but also full of trepidation: they do not remember the sun, and fear today might be a disappointment.

"It was all a joke, wasn’t it?" [William] turned to the other children. "Nothing’s happening today. Is it?"

When Margot protests, "But this is the day, the scientists predict, they say, they know," William, furious to be contradicted, grabs her and orders the children to throw her in the classroom closet and lock the door. He is angry at being challenged by Margot, who is a classroom nonentity, and he seems especially angry because she has already had time in the sun, back on Earth, so she can't understand what this day means to the Venusian children. He wants to punish her for her past history on Earth and for daring to contradict him in front of the others. The children throw Margot into the closet, as directed, and Margot screams and cries and flings herself against the door, but they leave her there, uncaring.
The sun does, in fact, come out, and the Venusian children spend an awestruck two hours in total ecstasy, running outside, warm and dry, finally free of the rain and able to see the sky. Only when it starts to rain again do they remember Margot, locked in the closet:

"Will it be seven more years?""Yes. Seven." Then one of them gave a little cry. "Margot!" "What?" "She’s still in the closet where we locked her." "Margot." They stood as if someone had driven them, like so many stakes, into the floor. They looked at each other and then looked away. They glanced out at the world that was raining now and raining and raining steadily. They could not meet each other’s glances.

They feel awful shame at what they have done, and Margot, no longer screaming and crying, is once again passive and silent when the children unlock the closet door and let her out.