In Chapter 26, Scout is confused about Miss Gates' lack of consistency: while she claims to despise Hitler's horrific abuse of the Jews, she welcomes extremely biased treatment of the black people in her own community.
In the schoolroom, Miss Gates is a respected authority figure. When she cautions her students against the same type of prejudice Hitler entertains, they take her words to heart. To her students, Miss Gates is a trusted source of wisdom and the very epitome of honor. It is not surprising, then, that Scout becomes confused when she hears her teacher railing against the supposed arrogance of black people. According to Miss Gates, "it’s time somebody taught ’em a lesson, they were gettin‘ way above themselves, an’ the next thing they think they can do is marry us."
Scout is so shocked by what she overhears that she brings her concerns to Jem. However, Jem is still too traumatized about the recent events surrounding the trial and Tom Robinson's death to adequately address Scout's concerns.
Basically, what confuses Scout about Miss Gates' view of Hitler is that her teacher doesn't see a connection between Hitler's type of prejudice with that of her own. For her own part, Scout doesn't understand how her teacher can rationalize her own ugly attitudes. Also, very likely, Scout finds it difficult to accept that an upstanding teacher can harbor such an obviously wrong perspective about others. To Miss Gates, white people are justified in their concerns; Hitler's prejudice, on the other hand, has more of a diabolical quality to it and must be disavowed at all costs. Miss Gates doesn't realize the inherent hypocrisy in her contradictory attitudes.