What was Calpurnia's fault in To Kill a Mockingbird?
According to Scout, it's Calpurnia's fault that she's got into trouble at school. Apparently, this is because it was Calpurnia who taught Scout to read, and Scout's new teacher Miss Caroline doesn't think that's appropriate.
A young woman with very little experience of being a teacher, Miss Caroline is somewhat out of her depth. Wanting to prove that she can handle a classroom full of kids, she's anxious to assert her authority, and she feels that this authority has been undermined by Scout's already knowing how to read and write. As far as she's concerned, it's her job to teach Scout these things.
Scout has just had a fight with Calpurnia over Cal Cunningham's table manners. Scout is horrified by Cal's uncouth behavior at the dinner table, whereas Calpurnia chides the young lady for what she perceives as her snobbery.
As a result of their argument, Scout isn't in the best of moods and blames Calpurnia for getting her into trouble at school with Miss Caroline because it was Calpurnia who taught her how to read.