Genshi Shoujo Urara

Yuu, who was like a hero at school because of his speed, has his ife completely changed by a prolonged hospitalization. Initially, classmates who came to visit gradually stop showing up, plunging him into the depths of despair. Despite being in despair, one day during an examination, Yuu suddenly time travels back to the primitive era... where wild action in a new world is depicted.

(Source: Natalie, translated)