There is no literal mockingbird killed in this book. Many see Tom Robinson's death as a symbolic death of a mockingbird because he was an innocent victim.
Innocence cannot be completely preserved in a society which is stained by evil. That is our society as well as the Finches.
However, I would say that by the end, the children are very hurt by the outcome of the trial. Their experience of looking at the difference between reality and perception leads them to their final defining moment with Boo. When they realize he is different than what they thought, I believe innocence is preserved.
If I had to pick between these two choices, I would go with innocence begin preserved. I think that Scout does get some bitter experience in the book, but the ending seems to say to me that she is still pretty innocent.
First of all, there's a happy ending of sorts. Mr. Ewell is dead -- he is punished for his bad actions and Scout can think that that will always happen just like it should. Boo Radley comes out and is a real person. This also is an example of things turning out right.
Finally, look at the ending words of the book. They really show an innocent way of looking at the world. Everyone is basically good and Daddy is there to watch over you all night...
I think that, in some ways, innocence is both preserved and replaced by bitter experience in this novel. The lesson that an adult Scout is trying to teach by telling her story is one of the importance of innocence. However, it is a concept that cannot be fully understood or appreciated without the wisdom that comes fro experience. To Scout, innocence was a given until she experienced the outcomes of the events of that summer. Seeing a group of men that she knew from the town ready to harm Atticus if he did not allow them to enter the jail, presumably to take and lynch Tom Robinson, gave her a first glimpse at the evil that lies in the hearts of all men. The outcome of the trial, which in her childhood wisdom she knew to be unfair, only serves to further that loss of innocence and respect for humanity. However, she has tempering influences as well. The scene when Atticus must kill the rabid dog, for instance, reminds her that some things that may not be pleasant are necessary, but that we must not hope or wish to be put into those situations. Boo Radley, as well, is an influence on her. He also acts as is needed to protect the children, but he is a simple man who has kept his own innocence through ignorance. It is these that provide her with the wisdom that she needs to maintain her appreciation and respect for innocence and to strive to teach those values to others through her narrative.