Is Scout a boy or a girl?

Scout is a young girl. She is around six years old at the beginning of the novel.

Scout (Jean Louise Finch)

The narrator and main character who begins her story at almost six years old. A rebellious tomboy, Scout has a fierce disposition toward any who challenge her, but at heart she believes in the goodness of people. Scout reacts to the terrible events of the book without losing hope in humanity.

But if she's a girl, why does Jem whisper to Scout, "... just hold your head high and be a gentleman" ? This is mentioned in Chapter 11, where Mrs. Dubose makes fun of Atticus for defending Tom Robinson.

Scout is a girl and one way to understand it is when Mrs.Dubose says,"what are you doing in those overalls? You should be in a dress and camisole, young lady!"