What is the main idea of the book "Speak"?

There are several themes in the book, Speak. Two of the biggest themes are about social casting and about keeping secrets. 

The first, about social casting, runs throughout the book in characters like Heather and Andy Evans. Heather longs to be popular, and Andy already is. Melinda is shunned by the popular people. The casting of the social strata is very important in Melinda's high school. 

The other theme, about keeping secrets, is what the book is titled after. The main character, Melinda, will not speak about the horrible event she experienced over the summer. Because of this, she turns inward and becomes an outcast. Her secret changes her, and changes the way people respond to her.