Based on the adult manga by momi.

Nyarko and Kogorou lately look so familiar, because in the club's activities there are only two of them alone. Many suspect Kogorou's and Nyarko's relationships about a pervert. So the Principal called them both to make sure. Having made the Headmaster believe that there was no connection between them, they began to continue the pervert thing that they kept secret.

(Source: Nekopoi and Has Been Translated)