what motivates the students to do what they do to Margot?

Th students, especially William, envy Margot because she is the only one of them who can remember seeing the sun. All the rest arrived on Venus when they were two. They are nine now, and the sun only comes out for about an hour once every seven years. It is about to come out again, but the last time they would have seen it, they would have been two: too young to remember.
Margot, however, only came to Venus five years ago. She remembers what the sun was like in Ohio when she was four. She tries to tell the other students about the sun, but they don't believe her.
Because she is depressed about being on Venus, she doesn't want to join in and play with the other children. She keeps herself apart. This also makes the children angry, as it seems to them as if she thinks she is better than they are. Therefore, right before the sun comes out, they lock her in a windowless closet.