Summer answers this question in part 3, which is on page 119 of my copy of this book. She admits that she sat with him on the first day because she felt sorry for him. She realized that with his strange face, this boy was in even more need of a friend than most new kids.
As Summer got to know August, she stopped sitting with him because she felt sorry for him, but continued to sit with him because she thought he was good company. She discusses how August is not like the other kids in the fifth grade who suddenly consider themselves too busy play games, preferring to "hang out" and "talk" over recess.
Summer proves her loyalty to August at Savanna's Halloween party, when she leaves after Savanna tells her that her chances of being popular and part of the "in crowd" would improve dramatically if she stopped hanging out with August.