The Drifting Classroom, Vol. 3 — Kazuo Umezu

As sixth-grader Sho fights for his life in the buried city that once was Tokyo, his cries for help travel across time and reach the ears of his mother in the distant past. But how can she save her son now? Back at the school, as their resources dwindle, the students revert to savagery and despair. Suddenly, a dying student staggers in from the wasteland, bearing signs that life may exist somewhere in their new world. Together with the last surviving adult, the children march into the desert...but are they ready for what they find?