Himitsu Kichi ni Aijin

An invitation to a secret base suddenly arrived for Tetsuji, who is called "Mambo-Sensei" by the students. I was suspicious, but when I arrived, there was a strange young man there. "I will answer your questions if you have sex with me once". The aim of the one who boldly invited me is...!?
(Source: Pixiv Comic, translated)
Note: Chapter count includes an extra.