Everything you need to understand or teach The Huntsman by Douglas Hill.
Although The Huntsman is basically the tale of a teen-age boy's courageous quest to rescue his foster father and sister from their alien captors, the past Hill creates for the events in the story inspires thought about human characteristics and values. Young Finn Ferral's environment is the world some three hundred years after a global fire.
The story of the "Forgotten Time," when billions of people inhabited the earth, is passed on to Finn by his foster father, Joshua. Obsessed with power and wealth, unmindful of the damage they were doing to the natural environment, those people had finally wreaked almost complete destruction on the planet. A portion of the world's population survived, but they were forced to scavenge among the ruins.
About a century following the catastrophe, aliens invaded the planet, slaughtering or capturing all human beings who attempted even friendly contact. Eventually, only a few thousand humans...