What is the author's purpose?

The author is writng from a ninth grader's point of view in the book "Speak." The author, Ms. Anderson tries to show the loneliness of a student who called the cops and busted up a summer party. Melinda, the main character, is beng avoided by her frinds. Rachel Bruin is Melinda's ex-best friend. Melinda would really like to tell Rachel what really happened the night of the party but she can't.