Students are denounced as "fags", "queers" and "lezzies". In some cases they are threatened and beaten up, and forced to leave school early.
An alarming number of these victims of homophobic bullying become suicidal, according to research. Although it leaves deep scars, it was a topic that was for many years taboo in Irish schools, but more recently steps have been taken to stamp it out.
A video aimed at Irish teenagers, Stand Up! Don't Stand for Homophobic Bullying shows how the subject resonates with Irish teenagers.
Since it was released two years ago, the video has been viewed 1.3 million times, making it the most popular Irish ad ever on YouTube.
A new book, Bullying in Irish Education, draws together research on the subject. A chapter on 'Homophobic Bullying' by Stephen Minton shows the extent of the problem in Ireland.
According to the book, surveys found that almost six in 10 Lesbian Gay Bisexual or Transgender (LGBT) people, and more than half of current schoolgoers, had suffered homophobic bullying in school.
More than half reported having been called names related to their sexual orientation by fellow students, and 8pc were even taunted by members of the school staff.
A quarter of all respondents, most of whom are now adults, said they were physically threatened at school because they were thought to be gay.
The experience of one 23-year-old, chronicled in the book, showed the effects of this type of bullying. He says: "I left school because of the hurt and suffering I got in school and the teachers didn't care.
"I think teachers took the attitude, 'well they call him gay, and he probably is gay, so why should we step in, because they aren't saying anything wrong'.
"This happened even though I hadn't come out at school. I was forced to leave at my Junior Certificate due to the abuse I got . . . (I was ) jumped on, and called 'puff' and 'queer'."
A study of the mental health of gay and lesbian people in 2009 showed:
* Some 86pc of respondents had suffered from depression.
* Of those surveyed, 27pc had self-harmed and an alarming 18pc had attempted suicide.
According to Stephen Minton, LGBT people suffer these effects not because they are part of a minority, but because they are part of a group that is at risk of abuse.
A more recent study of fifth-year students showed that 66pc of non-heterosexual females had been bullied in the previous two months.
In the book, he quotes a woman who attributes her suicidal feelings partly to being bullied as a lesbian at school. Michael Barron of the campaign group BelongTo, which represents LGBT people aged between 14 and 23, said: "Homophobic bullying is still prevalent in Ireland. It has terrible effects on peoples' educational attainment.
"Research shows that as many as one in 20 students leaves school early as a result of homophobic bullying. All the figures show it has a severe effect on mental health."
Experts in the field believe that ignorance about the topic exacerbated the problem.
Dr James O'Higgins Norman, a leading researcher on bullying at Dublin City University, says: "Absence of any teaching about sexual orientation contributes to homophobic bullying among students in Irish schools.
"Bullying of this type makes students feel alienated and isolated. They find it difficult to concentrate, and start missing school. An added stress factor for gay and lesbian students is coming out."
Until recently if students were given sex education classes in school – and there is evidence that in some schools it is barely taught at all – it was heterosexual. But that is now changing. Students are now being taught about "Growing Up LGBT" as part of Social, Personal and Health Education.
Dr O'Higgins Norman says: "Ten years ago, even to ask questions about homophobic bullying in schools was very difficult.
"Although things have changed, there are still a lot of teachers uncomfortable about LGBT issues, particularly men in all-boys schools."
The Stand Up! campaign to prevent homophobic bullying is based around the idea that friends should help victims.
Michael Barron says: "Our research showed what really mattered was friends giving support."
The latest anti-bullying guidelines for schools now have to deal with homophobic and transphobic bullying.
Bullying in Irish Education, edited by Mona O'Moore and Paul Stevens, is published by Cork University Press.