Tone refers to the author's attitude toward the characters, the subject matter, and other elements of the story. This can be especially difficult to suss out when the author doesn't tell us in interviews or essays or other bonus material. How can we know how the author feels about these things? Well, we can't, but we can make a few guesses. For example, even without hearing a word from S.E. Hinton, we can infer that she has respect her characters, and wants to tell the truth about them – even though some of them do awful things, she has empathy for them and understands them.
Hinton's remarks in interviews support that guess and tell us about her intended audience. As you can imagine, whom she's writing for impacts her tone. Here's a little piece of an interview with Hinton:
Q: When it was first published, the realism of The Outsiders shocked a lot of reviewers, but readers embraced the book. Did that surprise you?
A: No, I was pleased that people were shocked when The Outsiders came out. […] I wanted something realistic to be written about teenagers. At that time realistic teenage fiction didn't exist. If you didn't want to read Mary Jane Goes to The Prom and you were through with horse books, there was nothing to read. I just wanted to write something that dealt with what I saw kids really doing. (source)
This explains why the story has the feel of an exposé – a piece of journalism bringing something hidden to light. In its blend of drama, action, introspection, and especially in its fearlessness, it has the feel of an article in Rolling Stone magazine, whose first issue happened to be published the same year as The Outsiders (reminder: 1967). As the quote above implies, Hinton is interested in showing the truth of the people who inspired the story, not sugar-coating things to preserve the conventions of popular young adult literature at the time.