The Paradise on the Hill (2002)

Blurb:
Kijima teaches at an all girl’s school. While all his friends think his job is pretty "sweet," dealing with the daily confessions of his adoring students is actually rather annoying. Luckily the new P.E. teacher, Ono, has taken the brunt of the girl’s attention away. When Ono approaches Kijima for advice on how to deal with the bold advances and confessions from his students, Kijima finds himself intrigued by the shy, timid teacher.

Source: www.anisearch.com/manga/873