In answering this question, I will discuss the ideas of bravery and loyalty. The way in which this group of teens rises to the massive hurdles and calamities that are thrown their way showcases these ideas in an exemplary fashion.
With regard to bravery, the group shows immense mettle by choosing to launch counter-attacks instead of just going to hide out in Hell. Once they have seen that everyone is being held at the showground, they could have simply retreated and lived for months or even years down in Hell. However, they choose to fight back, launching a series of espionage missions and sabotage. For example, they get the idea of blowing up a bridge near Wirrawee that is important to the logistics of the enemy's efforts in the area.
Days earlier, Ellie had been a typical teenager whose biggest concern was who she liked more between Homer and Lee. Now, Ellie and Fi are stealing a tanker and setting it on fire. Life has changed irrevocably and this group of teens bands together bravely to form a covert army, taking other lives to protect their own.
The theme of loyalty is beautifully showcased at the end of the story, when Kevin chooses to sacrifice his freedom, and possibly his life, to get Corrie to the hospital after she is shot. He makes a speech which is one of the most moving paragraphs of the book:
I don't give a stuff what's rational and what's logical. Corrie's my mate and I'm not going to dump her and run. It has to be me or Ellie, because we're the only drivers, and Ellie, if you don't mind, I want to do it.
The loyalty that Kevin shows to his gravely injured girlfriend is worth learning about because it is a shining example of how to behave in a crisis.