Scout harbors her own prejudices at the start of To Kill a Mockingbird. What are they and what teaches her to lose these prejudices?

At the opening of the book, Scout does indeed have her own prejudices; however, they could more appropriately be what Atticus later calls "blind spots." As a child, Scout sees the world in a very simple, rigid way and is ignorant of how things like race or class shape and influence the way people are viewed by others. For example, she becomes very angry with her teacher for reprimanding her after she goes into detail...

At the opening of the book, Scout does indeed have her own prejudices; however, they could more appropriately be what Atticus later calls "blind spots." As a child, Scout sees the world in a very simple, rigid way and is ignorant of how things like race or class shape and influence the way people are viewed by others. For example, she becomes very angry with her teacher for reprimanding her after she goes into detail about the Cunningham's financial situation. In Scout's mind, the Cunningham's poverty is simply a fact and it doesn't change the way that she feels about Walter or his father. This also influences the way that she views the conflict around Tom's trial and how it's changing Atticus' social standing. 

As one of the more prominent themes in the story, Scout's understanding of human complexity and contradiction changes over the course of the novel. Her first lesson in this comes when Atticus explains that Mr. Cunningham anonymously leaves goods in exchange for Atticus' services because he is ashamed of his inability to pay. Prejudice and bias are also essential to how the children view Boo Radley. In their mind, he is a mysterious monster or thrilling source of adventure. It never occurs to her to consider why she believes this or that he could be an emotionally fragile or mentally ill man.

Scout is repeatedly faced with situations in which her previously simple understanding of people is challenged. Each new situation, from confronting the crowd outside the jail to the conclusion of Tom Robinson's trial, brings her one step closer to a better-developed understanding of how complicated people can be.

It's never quite clear if Scout overcomes her own biases, but the final encounter with Boo suggests that she has changed. Although Boo has been a source of fear and excitement for the children throughout the story, by the end she unquestioningly accepts him and eventually walks him home. Her figurative embrace of Boo Radley suggests that even if she doesn't quite understand why people are different, she no longer feels the need to aggressively challenge that which she doesn't understand.