The Pact-

Jodi Picoult is sometimes a little too predictable for me, but this one actually surprised me just a tiny bit at the end. So, I'm forgiving her for the last book I read of hers. :) The Pact tells the story of a family who have been best friends for 18 years, lived across the street from each other, with their children growing up together. Everyone is delighted when the children, Emily and Chris, start dating... it seems like a storybook fantasty, until one day when they are 18 years old and Emily dies of a gunshot wound to the head, and Chris is the only one standing over her with blood covering his body.
As the story progresses it breaks your heart to watch the parents crumble after realizing they actually knew so very little of their children's lives.

http://www.jodipicoult.com/the-pact.html