The Spiral Arm (episode 1, season 1)

Wren Harper lives on an overcrowded Earth on the brink of apocalypse. There are just too many people. The answer lies 600 light-years away on Kepler; a planet more than double the size of Earth. For decades humans have been fighting another race for its control. Earth’s armies are depleted. So now 15-year-old cadets are sent to fight, trained along the way in vast combat ships. But why has Wren been chosen? She's small and geeky and not a fighter. Will she survive Kepler? Or will the training kill her first? This is a debut novella-length episode in a series full of cliffhanger endings.