The Drifting Classroom Vol 2

The Drifting Classroom Vol 2 by Kazuo Umezz. A Horror manga rated Older Teen, published in English by Viz Media in 2007.

In an instant, an entire elementary school vanishes and reappears in a desolate wasteland. With the adults falling apart, sixth grader Sho Takamatsu and his classmates must fight monsters, hunger and each other to survive. A legendary horror classic from Kazuo Umezu.