In the Fold (1996)

A bunch of young and impulsive space navy cadets make their first real space flight and realize that the attack they suffered was not a training mission. They lose their senior officers in the opening battle, then through luck and guile attempt to defend a helpless colony. They face the enemy alone and have the chance to save the world, and maybe to prevent the war? Can the cadets conquer the more experienced, stronger and much more evil enemy?