In the story, the children are mistreating Margot because she's different. What would be evidence to show Margot's difference from the other children?
Margot is described as fundamentally different from the outset of the short story. It is because of this difference that she is able to be targeted so easily by the other students. Bradbury points this out in many moments during the exposition of the story. His description of Margot reflects this:
Margot stood alone. She was a very frail girl who looked as if she had been lost in the rain for years and the rain had washed out the blue from her eyes and the red from her mouth and the yellow from her hair. She was an old photograph dusted from an album, whitened away, and if she spoke at all her voice would be a ghost.
Such a physical rendering of Margot helps to make her a target of the other students. When Margot speaks of what the sun actually does look like, she is derided and silenced by the other students, another textual example of how Margot is mistreated because she is different. Interestingly enough, Bradbury points out that another reason why Margot is targeted is because of the envy of the other students. Consider this description of Margot's own state in relationship to her going back to Earth:
There was talk that her father and mother were taking her back to earth next year; it seemed vital to her that they do so, though it would mean the loss of thousands of dollars to her family. And so, the children hated her for all these reasons of big and little consequence.
The children target Margot because of their envy for her. In this textual reference, Bradbury makes it clear that there are several reasons for Margot's difference and this lack of respect from students.