Duty after School

 or Afterschool War Activities is manhwa written and illustrated by Ha Il-Kwon.

Plot
Strange things are appearing from the sky, and the world is slowly dying. In Korea, there aren't enough military men to fight these enemies off, and high-school and college students have no choice but to join the military and help the others survive - at the risk of their own lives.

Official

 *  at Naver (Korean, Pay)
 *  at Line Webtoon