Ponyboy's assignment is to write a theme about "anything (he) think(s) is important enough to write about...it isn't a reference theme...(it is his) own ideas and...experiences".  The length of the assignment is "not less than five pages".  In the last lines of the book, Ponyboy reflects upon the deaths of Bob, Johnny, and Dally over the past week and decides "I could tell peopole, beginning with my English teacher".  He goes on to describe how he began writing his assignment:

"I finally began like this:  When I stepped out into the bright sunlight from the darkness of the movie house, I had only two things on my mind:  Paul Newmand and a ride home..." (Chapter 12).

This line happens to be the very first line in Chapter 1 of the book you have just read, The Outsiders.  The end of the book refers back to the beginning, and what the author is trying to tell us is that it is Ponyboy who is the fictional writer of the story.  For his English assignment, Ponyboy wrote the theme, or story, The Outsiders. 

Mr. Syme is Ponyboy's English teacher. The man is concerned that Ponyboy's grades are falling because he is normally capable of much more. The teacher offers Ponyboy a deal, he tells him that he can write an autobiographical paper on his own and if he turns it in he can have a chance at passing the class. Ponyboy becomes inspired at the thought of getting his story out of his head and onto paper. He ends up writing the story about the Socs and Greasers and about living with his brothers following the tragic death of their parents. The story ends up actually being the book, The Outsiders.