Gakuin Kyoushi Seito o Hitojichi ni Zettai Fukujuu [sadozima]

Description: 
The protagonist, Kurogane,
leads students as a teacher on the surface,
but behind the scenes, he has long protected people as an exorcist,
fighting against interdimensional invaders known as Incubi.

However, due to the cunning sorcery of a high-level Incubus,
the souls of his students become bound to lower-tier Incubi.

If an Incubus is harmed, the student connected to it is harmed as well—

Kurogane is forced into submission by the Incubi,
and faces the cruel choice of offering up his own body as “bait”…