This is one of the few books in our literature classes that students actually read cover to cover. They identify with the characters and the struggles they face as teenagers, and the story generally hits home. I can't imagine why a school would challenge this book. How can we expect students to become adults if we never allow them to hear, read and discuss adult issues?
We have recently introduced this book as a 10th grade book, and it was mostly because Chris Crutcher came to speak to our students, so we read this book across the 10th grade before his visit. The students LOVED the book because it deals with real-life issues. More students in our schools than we would like to admit are dealing with tough issues like rejection, abuse, prejudice, finding their independence, and less than loving home lives. This book deals with the issues facing Sarah and Eric in a beautiful and honest way. Not every story ends happily, and students appreciate a little reality and less Happy-Ever-After. Of course, it's nice when positive things can come out of the ugliness as it does for Sarah. I can't imagine why any parent group would oppose this book when what our youth is listening to on the radio and their I-pods, watching on TV, and talking about in the hallways is so much more tragic. They are simply in denial.
Good luck with your challenge!