After-school war activities

One day, an unknown sapphire blue spherical life form suddenly appeared in the world, causing harm to mankind. Medium and large spheres can hurt people, while small spheres can attack people. In a crisis situation, high school students across the country receive military training and are organized into military reserve forces. They begin to fight with guns in various regions across the country. The task of the reserve soldiers is to defend and destroy small spheres deployed on the front lines and moving toward the city center. It is mainly a story about a high school student who becomes a soldier and starts fighting with a gun.