Owari no Seraph: Ichinose Guren, 16-sai no Catastrophe

"Hey, Guren? Do you think... when we grow up, we can still be together like always?" Just before the world fell to ruin because of mysterious virus that appeared on Earth which killed every infected human over the age of 13. Vampires conquered the surface—in that one last spring. Ichinose Guren had entered a Sorcery Training School in Shibuya. This school was full of elites from famous families in the world of sorcerers. As he himself was of modest origin, Guren held a great ambition in his heart, even despite being labelled as trash. But among his classmates, a girl appeared who introduced herself by the name of the fiancée he had traded vows with a really long time ago... In that world slated for ruin, the boy sought power; and the girl sought power as well. A magic academy fantasy story begins! (Source: owarinoseraph.tumblr, edited)