What is the irony of miss gates words about the holocaust?

In Chapter 26, Scout's class gets into a discussion about Hitler and the persecution of the Jews. Her teacher, Miss Gates, speaks at length about how the German dictatorship allows for the Jews to be persecuted by a prejudiced leader, but she claims that in America, "we don't believe in persecuting anybody." Scout finds Miss Gates hypocritical because she remembers that on the day of Tom's trial, she overheard Miss Gates say that she thought it was, "time somebody taught them a lesson, they thought they was getting' way above themselves, an' the next thing they think they can do is marry us." "Them" meant black people.