It was challenged at the Eleanor's George Washington Middle School in 2001 due to objections to the focus on gangs and gang fights, drug and alcohol use, violence, and offensive language was used commonly throughout the book.
When Was "The Outsiders" Banned?
When was the outsiders banned
I wasn't able to find the exact date but in the American Library Association website, it is only mentioned that it is amongst the top 100 challenged and banned books between 1990-2000.