18+ Tanetsuke Kyoushitsu: Ima kara Minna ni Sensei no Tokunou Seishi de Harande Moraimasu

A homeroom teacher doing one of society's greatest jobs:
educating our youth and securing their bright futures, and in effect, the future
of all generations. In this case though, society needs MORE generations!
The population is in decline, so onegai sensei, his duty... is to
splash his seed into the High school girls' wombs!

(Source: dlsite, edited)