Barbara Covett, the friendless lesbian character in Notes on a Scandal, develops an unhealthy obsession with a female colleague and later threatens to expose her relationship with a pupil.
Dame Judi Dench has been nominated for an Oscar for her portrayal of Covett in the film, released today. But Sue Sanders, co-chair of Schools Out, believes that such characters reinforce prejudices against gay staff in schools.
"Why aren't lesbian and gay teachers out in schools?" she said. "It's because of homophobia, stereotypes and myths, which this film only adds to.
The film will stir up talk about lesbians being predatory, feeding that prejudice. It's yet another image that isn't helpful."
Tony Fenwick, an English teacher in Hertfordshire, believes that such images will make it increasingly difficult for gay teachers to come out at school.
"The film portrays lesbians as bitter, twisted, psychotic people," he said.
"It is potentially another weapon in the armoury of the bigot. It is going to make teachers a lot more apprehensive about coming out to staff or pupils and about how they interact with colleagues."
Ms Sanders contrasts Notes on a Scandal with the recent series of Celebrity Big Brother, where televised incidents of bullying generated a broader national discussion about racial stereotypes.
She said: "Big Brother was a gift, to allow us to look at our own prejudices. This film has similar potential. It gives teachers a real chance to examine the diversity in our community. But I suspect it won't be used. Schools are still frightened of the issue."
Schools Out estimates that there are more than 25,000 lesbian, gay or bisexual teachers in Britain, but 99 per cent say that they are afraid to come out at school.
Mark Jennett, author of government guidance on tackling homophobic bullying at school, said: "There is this feeling that if a gay teacher talks about their relationship at school, it is bringing sex into the classroom. But gay relationships are like heterosexual relationships. If a straight teacher mentions their husband or wife, you don't start wondering what they do in bed."
This month is lesbian, gay, bisexual and transgender history month. It will be marked nationally with more than 300 events. These include a one-day conference on the impact of lesbian and gay issues on primary pupils, to be held in London tomorrow.
A lesbian mothers' group, Out for Our Children, runs a website to help nursery and primary school teachers who have children of lesbian parents in their class.
Mr Jennett hopes that the LGBT history month will encourage schools to tackle homophobia with the same seriousness as racism or sexism. He said:
"Children are used to talking about ethnicity, gender, prejudice and stereotypes at school, but sexuality is something we really cannot mention.
The more homophobia is hidden, the more it is seen as taboo."
He will address a one-day conference on lesbian and gay issues, to be held by the NASUWT union in Birmingham tomorrow.
Chris Keates, general secretary of NASUWT, said: "Homophobic bullying is one of the tools used to harass and make life difficult for teachers. But often it's just seen as people trading insults, and isn't taken as seriously as it should be."