It seems likely that Dashka Slater focused the first part of her book on Sasha because Sasha was the victim of the actions that Richard did on the bus that day. While Slater clearly wants readers to have sympathy for both Sasha and Richard, Sasha is still clearly the overall victim of what happened on the bus. Starting with Sasha is a way for the reader to get to know the victim before the crime is described in detail. It's a way for Slater to help the reader enter the story through the perspective of the person who is completely innocent.
Richard is also only a teenager, and his crime was meant to be a prank—but it caused lasting damage to someone. It was cruel. Richard is more difficult to empathize with than Sasha is. Because of that, his story isn't as good a point-of-entry for most readers. Instead, readers can get the context of Sasha's life before meeting the person they know changed Sasha's life.
Sasha's story also helps provide context for readers who aren't familiar with terms like "genderqueer." It helps them understand the rest of the book better to read through the explanations and definitions at the beginning. Since these terms and experiences are linked to Sasha's story, the author is able to introduce them earlier in context by beginning with Sasha.