Kotsugai no Kensei ga Shi wo Togeru: Noroware Seija no Gakuin Musou

Al is a holy knight, a warrior charged with protecting the innocent from the hordes of ravenous monsters terrorizing the lands. He was, anyway... After being bitten by an undead creature, Al dies–but not for long. Al awakes as an undead, cursed to live an undying life as a mindless monster. However, unlike ordinary undead, for some mysterious reason he retains his mind and sanity. Three hundred years later, flesh rotting off to reveal a living skeleton, Al meets Astrantia. As a descendant of a witch, Astrantia offers to restore his body on one condition: he must agree to become her knight and help her put an end to the unholy curse once and for all. Alongside Astrantia, Al enters an academy for training saints—teams of holy knights and holy women who fight to rid the world of the undead. Al must hide his identity as an undead—the very thing everyone in the school seeks to eradicate—in order for Astrantia to succeed and keep her word. Keeping his secret under wraps proves to be as difficult as hiding his lack of flesh but, if there's one thing he can't hide, it's the strength he's been honing for the past three hundred years! (Source: J-Novel Club)