In many ways, To Kill a Mockingbird recalls Sinclair Lewis's Main Street, a novel which portrays Smalltown U.S.A., especially in the Midwest. Harper Lee's novel presents a very typical view of the small, Southern town in all its kindnesses, attitudes, and religious hypocrisy. Having read both novels and having lived in both sections of the U. S., it is humorous to find the parallels in town after town.
I have always re-read TKAM every few years since reading it in high school; I feel like every time I pick it up I learn something new about the characters and about myself.
Now that I'm a teacher, I had the joy of teaching it to my freshmen for the first time this year; i was so excited that they, too, loved the book.
Personally, I would say that TKAM has, in fact, changed me to some extent. Every time I read it I'm reminded of the evils and perils of being judgemental and/or racist. This book reminds me that we have a responsibility not just to ourselves and those closest to us, but also to our communities at large.
I agree with the fact that the question as stated does assume there was a change, but as a teacher I tell my kids if your asked to answer a question like this it's not because the teacher wants you to say, "I didn't change." So find a way to write convincingly of something that did change about your thinking and use details from the book to support your answer. Such as, "After reading the novel To Kill a Mockingbird, my understanding of discrimination changed in that I know there are many different types . . .
To say that you have learned that African Americans used to be treated badly does not address the issue of how you have changed. Your teacher wants to know how the book and the issue of injustice, not just toward people of a different skin color but also toward people who are mentally disabled, have affected you. It would be better for the question to ask whether your opinions or attitude or thoughts have changed at all. That is, the question as it is assumes that after having read To Kill a Mockingbird, you have somehow been changed, and it is possible to read into it that the question assumes that you have changed for the better.
But this is 2009. Young people today don't have the same attitudes toward different races that were prevalent in earlier generations. It is conceivable that everyone in your class agrees that Tom Robinson was framed and murdered. Maybe the best answer you can give is that it didn't change you at all.
You can answer what understandings you have after having read the book. But I can't help you with that. Only you know what you have learned and become to "understand" from your reading.
I'm not your teacher and so I don't know what answer WOULD be acceptable, but I don't really think the answer you have given here would get you a very good grade.
My reasoning for saying this is that the answer you give above is not much of a revelation. In other words, you probably (if you have studied United States history in any way) should know by now that there was this era where blacks were treated badly.
Perhaps if you were able to say somehow that the book had helped you understand why or how the bad treatment happened, or if you say it allowed you to understand better how it would have felt to live in those days that would be better.