Set during the Depression in a small Alabama town where a black man is accused of raping a white woman, its exploration of racism, injustice and discrimination has placed it among the most banned or challenged works of literature in the United States, according to the American Library Association.

In 1985, its inclusion at a Park Hill junior high school in Missouri was challenged because of profanity and racial slurs. The same year, the book was retained on a supplemental eighth-grade reading list in the Casa Grande Elementary School District in Arizona despite protests by black parents and the National Association for the Advancement of Colored People that it was unfit for junior high use, the A.L.A. said.

The Biloxi School Board’s decision spread online and outside the state. Many saw it as a lost opportunity for students to have the frank conversations that generations of Americans need to have over their country’s history.

“In a state like Mississippi, where we continue to deal with racial injustices and discrimination even today, it is critical that our students have the opportunity to engage on the themes presented in To Kill a Mockingbird,” the American Civil Liberties Union in Mississippi said.

Arne Duncan, the former secretary of education in the Obama administration, and others on Twitter, welcomed the reinstatement. “Good news!” he tweeted.