Houkagogakari

"Kei Nimori, the Houkago"
One day, Kei Nimori, a sixth grader, finds his name scrawled on the blackboard, along with the mysterious title. That night, at the stroke of midnight, he is in his room when he heard the sound of school bells, and a portal slides open to reveal his school, but in another dimension, surrounded by darkness, the "Houkago." Monstrous beings, called "anonymous wonders," inhabit classrooms throughout the school. Six boys and girls are summoned to the Houkago to observe and record the nature of their assigned monsters. Kei, who is good at drawing, takes up a brush to capture "Makkaka-san," a monster lurking on the rooftop.......
(Source: Dengeki Bunko, translated)