Aster's Good, Right Things by Kate Gordon.
Blurb: Eleven-year-old Aster attends a school for gifted kids, but she doesn't think she's special at all. If she was, her mother wouldn't have left. Each day Aster must do a good, right thing -- a challenge she sets herself, to make someone else's life better. Nobody can know about her things, because then they won't count. And if she doesn't do them, she's sure everything will go wrong. Then she meets Xavier. He has his own kind of special missions to make life better. When they do these missions together, Aster feels free, but if she stops doing her good, right things will everything fall apart?
Reading Level: Years 6 - 9