How does Scout lose her innocence due to exposure to prejudice in Harper Lee's To Kill a Mockingbird? What quotes support this?

To lose one's innocence is to become aware that the world isn't the bright, happy place one once thought it was; it is instead full of evil, and in Harper Lee's To Kill a Mockingbird, most of that evil comes in the form of racism.

It takes Scout quite a while to internalize the meaning of racism and its consequences. Once she does, she fully understands the extent of evil in the world and loses her innocence. It is when Scout comprehends the meaning of Mr. Underwood's editorial, published in The Maycomb Tribune soon after Tom Robinson's death, that Scout fully understands the consequences of racism.

Towards the end of Chapter 25, Scout notes that Maycomb had been interested Robinson's death for all of two days. It is during this two-day period that Mr. Underwood writes a very enlightening editorial in which, according to Scout's narration, Mr. Underwood "likened Tom's death to the senseless slaughter of songbirds by hunters and children," especially since it was a cripple who had been killed. It is the use of the word senseless that gets Scout thinking. Scout wonders why Mr. Underwood can call Robinson's death senseless when "Tom had been given due process of law to the day of his death." Scout further reflects her belief that he had been "tried openly and convicted by twelve good men and true." It is at this moment that Scout realizes the beliefs she had held about the fairness of Robinson's trial simply were not true.

She finally realizes that Robinson had been judged to be guilty, well before his trial, simply because he was an African American being accused by a white woman. Therefore, Robinson had not been convicted by "twelve good men and true"; he had been convicted by twelve men who had not looked at the facts of the case objectively but rather through their own hateful, racist eyes. Furthermore, Mr. Underwood had commented on Robinson's crippled state because Mr. Underwood knew beyond a doubt that it was his crippled condition that proved Robinson innocent: a man with a useless left arm and hand simply could not hurt a woman on the right side of her face.

The most poignant passage that reflects Scout's new enlightenment about the evils of the world and her loss of innocence is the following:

Then Mr. Underwood's meaning became clear: Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men's hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed. (Ch. 25)

Scout's enlightenment about the evil prejudices in the world is further reflected when she realizes her third-grade teacher is a hypocrite, as Scout reflects in her following comment to Jem:

Jem, how can you hate Hitler so bad an' then turn around and be ugly about folks right at home--. (Ch. 26)

In the above, Scout is commenting on the fact that Miss Gates, her third-grade teacher, spoke out in class against Hitler's unjust treatment of the Jews after also having approved of the unjust treatment Robinson received during his trial. Scout's revelations about the hypocritical natures of people shows us that she is now fully aware of the evils in the world, which also shows us she has lost her innocence.