Is this a good thesis statement: "Harper Lee uses the discrimination of characters in the novel to show the injustice of their attackers"? I'm...

I can't answer all of this question because I'm not a literature teacher and haven't read the book myself in the last 10 years.

However, I do have a couple things to say that I think can be helpful.

First of all, I'm not sure that you have to write something that isn't obvious.  If you are still in 9th grade, or even if you're in 10th, you're probably not expected to see beyond the obvious.  More importantly, I think this book is sort of an obvious one anyway, with its not very subtle message of tolerance for people who aren't like you (racially or in terms of "weirdness" like Boo Radley.

Second, whether obvious or not, the sentence you've provided has some word use problems.  The phrase "discrimination of characters" is not one that sounds normal and "injusticeness" is not a word.

I think what you're trying to say is "In this novel, some characters are discriminated against.  Harper Lee tries to use what is done to them to show that their attackers are unjust."