In the novel, Andy tries to rape Melinda a second time, whereas in the film, he physically assaults her instead.
The film mentions the story taking place somewhere in the Midwest. The novel takes place in Syracuse, Anderson's hometown.
In the novel, she is raped outside in the trees at the party. In the film, she is raped in Andy's car.
In the book, when Andy tries to rape Melinda a second time, she holds a shard of broken glass up to his neck and says "I said no," but never actually hurts him. In the movie, she uses the glass shard and throws aerosol in his eyes.
In the film, all that Melinda writes on the bathroom stall is about the "exchange students ruining the country," whereas in the book, she writes about Andy Evans and other girls add to the writing.
In the film, Ivy hangs out with the Marthas; she doesn't in the book.
And in the book Melinda goes to Heathers house and in the movie she doesn't.
In the book there is an assembly at the beginning and in the movie there isn't one.
In the book she said on the bus in the beginning 'I'm the only one sitting alone' but in the movie Heather sits with her.
She never went to the mall and sat with Ivy.
In the book she gets sick and stays home and in the movie she never gets sick.
I know there are many more little details but these were all I could find.
