This is an amazing book series that was written by an Australian author and has just been released as a movie here, I haven't seen it yet but have heard great things. I first read the series in high school, it was one that my sister recommended to me. It is seriously brilliant.
It is about a group of 7/8 school kids who go camping during their summer holidays for a week and when they go home they find out their entire country has been invaded and war has been declared. Their families have all been captured and are being held at the local show ground.
From there it is just about how they try to survive and help save their families. It is a series of 7 books which are only short, I am rereading the first one before seeing the movie and in 24 hours I have gotten a bit over half way through.
I just wanted to recommend this to everyone and if you get the chance try looking for the books.