Houkagogakari

A sixth grader named Kei Nimori discovers his name written on a blackboard with the strange title "Houkago." Later that night, he hears school bells at midnight, and a portal appears, leading to a dark alternate version of his school. This realm, the Houkago, is filled with monstrous creatures known as "anonymous wonders" that occupy the classrooms. Kei and five others are brought there to observe and document the monsters they are assigned. Kei, skilled in drawing, uses a brush to sketch "Makkaka-san," a creature found on the rooftop.