Dr Stuart Newton, head of Selsdon high school in Croydon, south London, was horrified when a fax landed on his desk inviting people to a cabaret entitled "Puppetry of the Penis".
The show, launched at the Edinburgh Festival, involves two men manipulating their genitals into various shapes, including hamburgers, windsurfers and the Eiffel Tower.
The cabaret's producers described the performers - Simon Morley and David Friend - as "genital acrobats" who had trained for several years "in an ancient Australian art".
The production, now showing to sell-out audiences at the Whitehall theatre in the West End, is described as "a non-sexual show containing scenes of full frontal male nudity" and as being "perfect for office parties".
The poster was faxed to the school because one of its teachers was on th mailing list for OIT, a marketing firm which sends out information on London entertainment events.
Dr Newton said: "I was gobsmacked. If a pupil had seen the poster and told parents, very serious questions would be asked of the school. I don't believe in censorship and what people choose to do in their free time is up to them.
"Fortunately it came through to the bursar's office. But bits of paper can get where they shouldn't.
"People may think I'm over-reacting but schools are held responsible for the morality of the nation."
Dr Newton has agreed to let the matter drop after a full apology from OIT.
The company's account manager Richard Andrews said: "It's very rare for us to advertise anything controversial and we would never knowingly have sent this to a school. The person we sent it to had requested information on comedy events."
Mr Andrews promised the school would not be sent any similar material in the future.