This is what the book is about, since it's not on the product page:
Sugar is a sixteen-year-old student at Chesty B. Willis High School in sunny Barstock Township. She is an atheist in a very religious community, but this has not affected her life to this point, and so she ignores it, and lives her life quietly.
Her only friend (due to some kind of social disorder which makes meeting new people a torturous affair) is Tricia, the daughter of a very religious single mother. Tricia is pressured by her mother into taking part in a variety of abrasive religious activities that she secretly wants no part of. The most notable of these is the Barstock Christian Youth Fellowship.
The Fellowship is led by Byron, the fundamentalist son of a rather moderate preacher, who decides one day to use the school to spread his beliefs to everyone, whether they want them or not.
This is the final straw for Tricia. She takes a stand against him, and becomes the town pariah as a result. Sugar would like to help, preferably in a way that doesn't require talking to anybody, but soon learns that the only response to political activism, is more political activism.
Meanwhile, Sugar's social phobias are stressed on two fronts. First by Yui, the strangely forward student/shop girl from across town who is hell-bent on making Sugar her friend for some reason; and second by Joss, the boy she saw at the video store whom her peers are pressuring her into saying 'hi' to.
Thus, partisan politics take root in the small town high school. Everyone takes sides, and Sugar becomes the most hated person in town, when all she really wanted was to be ignored. Can Sugar find a way to reason with people that see her as literally evil?