The main way in which Margaret changes is that she is able to see that she can trust her own integrity. At the start of the novel she feels unsure of herself, in particular she is unsure of her social standing at her new school. By becoming friends with Nancy and the other girls, she feels she is part of a social circle. But she still defers to Nancy's leadership and still feels foolish about her choices and things she says and does, because the other girls sometimes make fun of her. For example: She pretends to like the same popular boys that her friends like, because when she says she likes less-popular boys, they ridicule her choices. Perhaps because she is a relative newcomer to the neighborhood and the school, Margaret seems to think she needs to behave more carefully than the others, so as not to lose her social position. In her more private moments she continues her conversations with God, sharing her worries and asking for help.
The other two girls also tend to defer to Nancy, who is the natural leader of the group and projects the most confidence. Nancy's dominance sets the tone for the girls' gatherings, and she frequently turns the conversation to issues surrounding puberty, such as their breasts being too small and the embarrassing films they have to see in health class, but also about boys. After two of the girls in her circle of four get their periods, Margaret feels awkward and jealous. She doesn't like how she is feeling and gets angry at God in her continued conversations directed at Him. She seems to want to strike out but doesn't know where to direct her anger.
When Margaret finds out that Nancy was lying about getting her period, she is confused and disappointed. Margaret realizes she has been foolish to let Nancy influence her to such a great extent, but also realizes Nancy is not perfect and makes mistakes. Something changes in their relationship at this point, and Nancy seems more humble and deferential to Margaret. Margaret knows she could hurt Nancy by telling the other girls about her lie, but she chooses not to, and seems to forgive Nancy for lying. This shows her maturity and self-awareness; and Nancy's response is to act as if Margaret is being a true friend. Both girls grow from this experience. Going forward, Margaret is more confident and even decides she will not need to talk to God as often, because she has figured out how to help herself a bit more.