A thesis statement makes a claim about an argument, and throughout the paper, the author supports their argument with specific evidence. In this assignment, Scout's maturation, perspective, and moral development are the main topics of the paper. The author's thesis statement could argue the importance of a moral education or the value of positive parental influence on their child's behavior. The thesis statement could also argue how one's environment affects their perspective on life and moral development.
Throughout the story, Scout is both positively and negatively impacted by her community members and environment. There is a pervasive prejudice throughout her community, yet Atticus raises Scout to be a tolerant, morally upright individual. As the novel progresses, Atticus teaches Scout various life lessons, which gives her additional perspective on life and aids in her moral development.
The following are good examples of thesis statements dealing with Scout's maturation, growth, and moral development:
- "Despite growing up in a prejudiced community, Scout listens to her father's advice, which significantly impacts her moral development."
- "Scout's perspective on life is significantly impacted by her environment, her experiences, and the way she was raised."
I am wondering if what you actually need to do is focus on the dynamic (changing) nature of Scout's character throughout the book. While a theme is generally considered to be a message or "moral" to the story that can be generalized to the human experience, the three examples you mention above are directly tied to changes in Scout's character in the novel. The incident at the beginning with Walter is part of Lee's characterization of Scout as an innocent tomboy; the example of her behavior at the tea is an example that demonstrates to us that some of Aunt Alexandra's teachings have started to sink in and that Scout has begun to grow up; and her perceptive treatment of Boo Radley at the novel's end shows us that she has achieved a depth of understanding about human nature that would've been impossible for the Scout of the novel's early pages. Perhaps this statement might more easily fit your examples: "Scout Finch changes a great deal throughout Harper Lee's To Kill a Mockingbird.