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.