Miss gates says,"over here we dont believe in persecuting anybody.persecution comes from people who are predudiced." Why is it ironic?

In To Kill A Mockingbird, by Harper Lee, Ms. Gates is Scout's school teacher.  This line is taken from the scene where she is teaching the class about Hitler and the persecution of the Jews.  It is ironic because Scout overheard her coming out of the courthouse after the verdict was over.  Ms. Gates had turned to Miss Stephanie Crawford and commented how it was about time someone taught the blacks in the community a lesson.  Here she is, trying to teach her students about how horrible discrimination and prejudice are, but she is no better than the people of whom she speaks.  Even though she is very young, Scout realizes how hypocritical Ms. Gates is, and even comments about it to her brother, Jem.  Jem becomes very upset with Scout and demands that she never discusses the trial again.