Rose is not really friends with Molly and Claire.
The first thing Melody tells the reader about Molly and Claire is that they are best friends who are mean to many other students and make fun of others. It's obvious that Rose is somewhat intimidated by them, because Melody is surprised at one point that Rose encourages her after she gets a bad grade in front of Molly and Claire.
Molly and Claire don't seem to attack Rose in particular. When they run into her at the aquarium with Melody's family, they ask if she went there with Melody on purpose. They see her as less than others because of her disability.
Near the end of the book, however, Rose is whispering with Molly and Claire when Melody sees them and wonders whether they're talking about her. Later, after they leave her behind, she finds out that they're embarrassed to have her on the team.