Why does Margot refuse to take a shower in the school shower room?

Although the story doesn't specifically explain why Margot wouldn't shower in the school shower room, the description of how she refused for a month, previous to the day of the story, to let the water touch her head, is part of the author's characterization of Margot. Margot is the only one of the schoolchildren in her class who knows what the sun looks like. All the rest have lived on Venus all their lives and were only two years old when the sun last appeared.
Margot is portrayed as a child who misses the sunshine desperately, so much so that she may be depressed or traumatized by the constant pattering of rain at the windows. The incident in the shower room, when she "clutched her hands to her ears and over her head, screaming that the water mustn't touch her head" is a powerful description of the intensity of Margot's psychological need for sunshine. The rain has become so emotionally damaging to her that her parents will be taking her back to Earth next year, even though that entails an expense of thousands of dollars.
Rather than feeling compassion for Margot because of her antipathy toward the rain, the other children hate her for it because it sets her apart as different from them. Knowing what we know about Margot and her fragile emotional state makes the children's cruelty in locking her up and preventing her from seeing the sun all the more diabolical. Margot's unwillingness to shower clearly communicates her psychological need for relief from the rain and how essential it is for her to enjoy the one day of summer that is coming. Although she is the one person who can probably most benefit from the few hours of sunshine, she is the one who is deprived of it by her bullying classmates.