18+ Ore ga Ojousama Gakkou ni "Shomin Sample" Toshite Rachirareta Ken

In a hidden school surrounded by mountains, young ladies of good families are taught everything a high class lady needs to know. They are also completely cut off from the outside world, keeping them sheltered and innocent. However, a problem has come up recently―more and more graduates are failing to cope with the modern world after graduation. To deal with this problem, they've decided to kidnap a male commoner student, and enroll him as a student. They hope that this will be a gentle way to introduce the girls to the outside world.

Kagurazaka Kimito is chosen for this dubious honor, mostly because he seems utterly average. However, the school leaders also seem to have concluded he's a homosexual with a muscle fetish. When Kimito hears that the alternative may be castration (to protect the girls' purity), he eagerly plays along with this misconception. Now he'll just have to survive the attention of many high class young ladies that are completely out of touch with the world!

(Source: MangaHelpers)