I imagine it was the screenplay of the 1963 version of the film. That is strange. While the screenplay sticks pretty close to the book, a book and a screenplay are different beasts. Did they show you the movie as well? Both the book and the movie are terrifying and sad. I can think of several reasons why they would teach it in school. 1) it's about school-age kids and 2) it's a cautionary tale, in a way, about what will happen if you don't listen to your parents and teachers.
I absolutely recommend reading it. I had a really unique experience reading and learning the book. lil, have you taught the book ever? I first read it as a sophomore at Catholic school. We learned a lot about the biblical references. The Character Simon was taught to
be the Christ like figure. Sacrificed, pure for the rest of the group etc. A year later I was removed from Catholic school and sent to public school. The teacher of my Junior year lit class taught the book as well. I despised this teacher. He knew nothing about literature and admitted to me that he got in to teaching for the summers off and to coach basketball. He wouldn't hear me on any of my interpretations of the book. It was a terrible experience and was the first time I realized that there could be a big disparity between educators.