It's not.
This quote is not in the book The Outsiders. However, it was said by S.E. Hinton, the author of The Outsiders. The quote can be found as the tagline for the movie version of the book. A tagline is the catchphrase for the movie. It is used for advertising purposes to get people hooked and interested in watching.
Chapter 1 of the book supports this quote when Ponyboy describes the Greasers and the Socs.