From Jana's TG Lists:

Sora Owada's a high school student who's quite fond of females. He's got a steady girl named Momoe but frequently sneaks into love motels with other young ladies. The name 'Willy' is his nickname for his favorite little phallic pal. One night Sora runs out of underwear and slips on a pair of panties Momoe left at his place. Imagine his surprise when he wakes up belonging in them! Yes, Sora's a girl. She's naturally stunned at her transformation, she even discovers that her speech patterns and mannerisms have become female, but her biggest shock is the realization that she's a virgin again! Before she knows it, Sora is attending an all-girl's school as Sorako. She makes new friends, weathers a rocky start as she rediscovers herself sexually, and generally begins to live the life of a normal girl.