There isn't a doubt about the fact that this book is for teens. The characters are pretty much all teenagers, stuck asking for permission to go camping and borrow cars and all that good stuff. Sure, an invasion happens forcing them to make it on their own, but their hormones are still pumping and that means crushes abound. In other words, for all the strangeness of the situation this cast of characters finds themselves in, they're still really relatable for teen readers—they still struggle with classic teen drama in addition to the war.
Shoring up this book's YA status is the fact that it's recounted by a teen. No million dollar words or tricky plot devices here; Ellie's just an ordinary girl trying to document their extraordinary experience.
This story is dramatic, and all the drama and tension happens because of an invasion on Aussie soil. It's because their families are taken hostage and their houses looted and their pets dead that these kids become guerillas: "When we did go into the culvert we travelled like snails rather than crocodiles or soldiers, crawling silently along" (19.6). Yup, the tension is officially high. So it goes when you come back from a camping trip to find your country invaded by foreign soldiers.