Via explains that she loved middle school because she could be Olivia Pullman there, not Via. In elementary school, everyone knew about August because he came to all the school functions with Mom and Dad. In middle school, however, not as many people knew about August. It was nice to not be known just as the girl with a "deformed" little brother.
Via's relief at getting to be Olivia and not having people know about August points to the novel's assertion that adolescence naturally includes attempts to define oneself in one's own terms.
In high school, it seems as though only Miranda and Ella know about August, and they don't talk about it with other people. They've all been friends since first grade and loved playing with August when he was little. In middle school, they were solidly in the middle of the social structure, and they were all thrilled to get into the same high school. Now, Via isn't sure what's going on in high school, as it's nothing like what she expected.
Even as Via enjoys being able to separate her identity from August's in high school, the suggestion that the social structure is difficult and shifting shows again that the process of growing up isn't linear or easy.