What would you say "To Kill a Mockingbird" is really about?

I teach this to my freshman students and I have had them ask me this. Of course, the main plot is Tom Robinson being falsely accused of rape. But the first twelve chapters or so are not even about that. We see cookie crumbs but the name Tom Robinson does not pop up until relatively late into the book.

I tell my students that the book serves as a foray into growing up and figuring out the evils of the world for yourself. But I am not so sure that is an answer they can digest.