If I were to answer your question on a test I would say that there is enough background information in the story to argue that it is indeed quite pessimistic. We have division of class, violence, drunkenness, death, elitism, pain, suffering, law-breaking: All among teenagers!

The only optimist perspective in the story is the end, when the main character decides to tell the story of his dead friends as a way to put an end-fire to his eternal life of violence. Other than that, yep, pretty pessimistic.

This book, despite all the tragedy in it, is more optimistic than pessimistic.  When trying to decide the tone of a book, it is important to look at the arc of the protagonist more so than the supporting characters.  Ponyboy is the protagonist.  He is a optimistic person himself.  He likes sunsets, he likes to daydream.  He is, as Johnny says, "gold."  Even though the poem says "nothing gold can stay," Johnny tells Ponyboy to "stay gold" and Ponyboy take the message to heart.

The main conflict of the book is between the Greasers and Socs.  However, from the very beginning, Ponyboy is able to cross the lines of the social groups.  He becomes friends with Cherry and he has a cordial relationship with Randy.  This suggests to us that there can be an end to this conflict.

At the end of the story, Ponyboy is able to conquer his own conflicts.  After Sodapop's outburst, he and Darry makes a commitment to get along and try to understand one another.  Ponyboy accepts Johnny's death, and sets out to tell the story of Johnny, Dally, and Bob, in order to help other kids out there like them.  This is all positive, and suggests forward movement for Ponyboy and for society.

The classic S.E. Hinton teen novel obviously has elements that are both optimistic and pessimistic. Certainly the ending is upbeat in spite of the deaths of Johnny and Dallas: The Curtis brothers are allowed to stay together as a family, and Ponyboy appears to be back on the right track, working on his essay in English--and perhaps beginning his career as a writer. The greasers and Socs seem to have earned a mutual respect for one another at last, and there is hope that the violence between them has come to an end. Needless to say, pessimism rules during much of the story: The greasers have little hope of ever rising above their social status, and the Socs seem to revel in their upper-class snobbery. The most pitiful of them all, Johnny, only suffers further horrors after being burned in the church fire; however, he dies knowing that he performed an act of heroism by saving the children from burning themselves. The doomed Dallas Winston suffered a similar fate; he took humor in the fact that he was being hailed a hero, but he resorts to suicide-by-cop after Johnny's death. The Outsiders may have its share of characters who look at life from the outside in, but it does offer hope that a change for the better is still possible.