Danshi Kokosei Dakedo Gal ni TS Shimashita

Junpei, a loner high schooler, became a gyaru after saving the god of gyaru from drowning. Through his mother's connections, he was able to transfer to an all-girls school, but it was stuffed to the brim with gyarus! Every day is a sensory overload for Shunpei, who's never even talked to a girl, let alone a gyaru.
(Source: Kadokawa, translated)