Ms. Marie Rose Napierkowski wrote a great summary of this book here on enotes.com (see source below). In the introduction, she lists many reasons why the novel was popular when it was first published; those same reasons are why it prevails today, so check it out! One of those reasons includes how Harper Lee, an adult, wrote the novel from a child's perspective. This is generally a very difficult thing to do because the adult's voice can intrude on the child's character/narration and devalue the credibility of the story. (The enotes introduction also describes other reasons.)
As far as why it is so widely read in schools, I'd say it has become a rite of passage. The themes that are dealt with in the novel include racism, tolerance, and courage during a very difficult period in America's history. The story takes place during the Great Depressionand deep in former Confederate territory. The novel emerged during the time that Civil Rights burst onto the historical scene. America was confused, in pain, and didn't know how to solve the deep-seeded issues facing her. To Kill a Mockingbird helped to open up the discussion in a non-finger-pointing way. Narratives are like hypothetical situations; one might be able to get a point across better through stories than through direct insults.
Many people in America were tight-lipped about saying anything controversial in social circles during the first half of the twentieth century. This book helped to bring the discussion of such difficult themes to the classroom, to the public, as well as to the home. It's almost as if it were a handbook from which to discuss patience, tolerance, and racism.
Today, I hear students throw the "R" word around very satirically. I usually take a day in class to discuss the differences between words such as racism and discrimination (among other similar topics) just so the students won't misuse the term in every day speech. Reading this novel in the classroom helps teachers to supplement that discussion in a safe environment before it gets out of hand.
People are prone to repeat history, right? Reading this novel is one step in helping us not to repeat what happened in the past.
When I teach a novel, it needs to speak to me personally. I read Mockingbird when I was in grade ten. Never has a book stayed with me for so long. Atticus Finch, although fictional, brings me a sense of hope which I pass on to my students. Doing the right thing in the face of overwhelming opposition, sticking to your guns and empathising before judging people are just some of the life lessons that Atticus brings to the table. I know these themes are old chestnuts but Harper Lee's simple narrative style is so effective, this book will never lose its meaning. Even today, when faced with a moral dilemma, I sometimes ask myself a defining question; what would Atticus do? I, of course, usually fall short but the bar is always set high.
The above discussion may have left out the most important reason that students study To Kill a Mockingbird.
According to my grandfather when he was my age, he walked up hill in the snow to and from school, and the students read books like Moby Dick, Light in August, Gone with the Wind, and Red Badge of Courage.
To Kill a Mockingbird has one thing that the above books do not have.