Based on the erotic game by Lune.

Meikoku Gakuen is a school where they will face the Dark Ages. At that time of darkness, all schoolgirls will feel very aroused so they can not escape the sensation of pleasure. There is only one way that can stop them from excessive lust in the dark ages is by giving sperm that already contain magic.

(Source: Nekopoi and Has Been Translated and Edited)