I teach fith grade and read this to my class each year. Every class has always loved the book, and can't wait till I read the next chapter. Both boys and girls enjoy the book and it sparks much discussion about implications of eternal life.
The story is beautifully written with great use of metaphor and simile to create feeling and draw the scene.
The book's theme is life and death. The Tucks are a family that has become immortal by unknowingly drinking from a hidden spring. Winnie sees Jesse, the Tuck's son, drinking and takes her away to explain why she should never tell the truth about the spring. That is, if everyone knew about it, it would destroy the meaning life.
A man who is trying to steal the spring, to use Winnie, and to share the secret of the spring is killed by a blow to the back of the head with a shotgun stock.
The book does not encourage Winnie breaking the law or disobeying her parents, it spend a few pages going over her tribulations about it. Winnie must sacrifice the trust of her parents to save the Tucks from hanging and the world from finding the secret of the water.