Tengoku no Saitai
Beings called the Abandoned have come upon the Earth. At first, they were thought to be Demons and Shinto Priests from Japan seemed to have knowledge about them and how to attack them; thus the Shinto War would begin and last for 12 years.
The end of the war was heralded by an unknown and unsung hero who found a way to bond with one of the Abandoned to create a powerful partnership called the Covenant, and made way for a time of peace. The Shinto Priests referred to this hero as a Fallen one; one with the unique ability to bond to an Abandoned.
The first few years after the war were tumultuous, though in time more and more Fallen became identified, almost entirely comprised of young teens, while the Abandoned became regarded more as pets than fiends.
As time went on however, evidence of a hidden enemy began plaguing the people of earth once more, an invisible enemy aptly named the Unseen. People seemed to simply drop dead, and only the Fallen/Abandoned seem to be able to defeat this enemy.
Governments around the world saw the need to train these youngsters for this purpose. Secondary schools that once only focused on education now have the resources to teach and train the Fallen to master their unique abilities and stop the Unseen once and for all.