You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
The teachers' criticism came yesterday hard on the heels of The Hobbit star Sir Ian McKellen's warning to Mr Key that his language was "careless" and could damage lives.
In an open letter to Mr Key the Post Primary Teachers Association's "Rainbow Taskforce" said the Prime Minister's use of gay as a derogatory term belittled a vulnerable group of young people.
The taskforce had been developed to educate secondary teachers about the challenges lesbian, gay, transgender and intersex youth faced in classrooms and on playing fields at schools.
The focus of the campaign was getting teachers to challenge the use of homophobic language, specifically the use of the word "gay".
"So we are saddened to hear the leader of our country not only use this homophobic language but then explain it away with, 'young people use it all the time, I don't think too many people would be offended by it'.
"You are perpetuating the harmful use of such a simple word," the teachers said.
Openly homosexual Green Party MP Kevin Hague said Mr Key's use of the word "gay" to tease a radio host last week relied on homosexuality being perceived as something "pathetic, awful, sad, terrible" or, as Mr Key claimed, weird.
Mr Hague said he was sure Mr Key did not set out to offend anyone "but the fact is that as Prime Minister when he uses the word in that way he is reinforcing that negative equation".
Mr Key used the word to describe a radio host's red shirt during an interview last week but yesterday refused to comment, saying he'd dealt with the matter on Monday.
Sir Ian, who plays Gandalf in the Lord of the Rings series and upcoming Hobbit trilogy, said Mr Key's language reflected badly on New Zealand.
"Mr Key should watch his language," the gay actor said in a blog post.
"I do hope John Key listens to his critics."
Last week: Reportedly saying English footballer David Beckham was "thick as batsh*t" and telling radio presenter Jamie Mackay his red top was "gay".
October 2011: In his haste to congratulate victorious All Black captain Richie McCaw after the Rugby World Cup final, Mr Key joins McCaw and International Rugby Board chairman Bernard Lapasset in an unusual three-way handshake seen by an international television audience of millions.
January 2011: Wearing purple and teal Rugby World Cup volunteers' garb Mr Key minces down a catwalk , flapping his hands and waggling his behind.
May 2010: Mr Key's flippant remark about having dinner with Ngati Porou, as opposed to their neighbouring iwi Tuhoe, "in which case I would have been dinner, which wouldn't have been quite so attractive", is picked up by international media who describe it as "a cannibal joke".
- Adam Bennett, NZ Herald