Mayo Chiki! Omnibus Vol. 1-3 (manga)

Sakamachi Kinjiro is a "normal" seventeen-year-old boy who happens to suffer from an abnormal fear of women, otherwise known as gynophobia. His phobia, which surfaces as an occasional nosebleed, has been kept more or less under control... until now.

A fateful visit to the school bathroom turns Kinjiro's world upside down, as he discovers the dashing and popular Konoe Subaru is actually a girl?! What's worse, the principal's cruel and domineering daughter, Suzutsuki Kanade, has every reason to keep this secret from leaking out, and makes it her life's work to torment Kinjiro, while attempting to cure his gynophobia.

Is Kanada simply sadistic or does she really want to help Kinjiro overcome his fears? It makes no difference to poor Kinjiro, because his life has just become a living hell.