The Third Day, The Frost by John Marsden

A tight-knit group of teenagers continue their guerrilla resistance after an enemy invasion, mounting daring raids and sabotage from the bush while struggling with fear, grief and the strain of leadership and relationships. As resources and innocence dwindle, they face increasingly dangerous missions and devastating personal losses that force them to confront the moral costs of survival. Told through the narrator's candid, emotional voice, the story traces their tactical planning, camaraderie and the heavy choices that shape who they become.