The children put Margot in a closet while the teacher was out of the room.

Margot’s teacher does not seem to really be on top of things.  She is barely aware of how much the other children are bullying Margot.  She interferes somewhat in the argument between the boy and Margot, but then she leaves the room.

The argument between Margot and the boy is about a poem Margot wrote describing the sun.

"Aw, you didn’t write that!" protested one of the boys.
"I did," said Margot. "I did."
"William!" said the teacher.
But that was yesterday. Now the rain was slackening, and the children were crushed in the great thick windows.
“Where’s teacher?"
"She’ll be back."

Any teacher knows not to leave a group of nine year olds, or children of any age, alone for any significant period of time.  When the teacher left, the kids were already stirred up about the sun coming out.  They had not seen the sun for years because it rained all of the time on Venus, and they were very excited to see it since it would only be out a short time.

One of the boys teases Margot by telling her that nothing is going to happen after all.  Margot gets very upset, but he presses on.

"Nothing, nothing!"
"Oh, but," Margot whispered, her eyes helpless. "But this is the day, the scientists predict, they say, they know, the sun…"
"All a joke!" said the boy, and seized her roughly. "Hey, everyone, let’s put her in a closet before the teacher comes!"

Margot is in the closet when the sun comes out, and she misses it.  The other children are horrified when they realize what they have done.  Even though they were bullying Margot because she was different, they did not mean to be so cruel to her.  For Margot, there would be nothing worse than missing the sun finally come out.