Fahma Mohamed first heard about female genital mutilation (FGM) when she was 13. Educated, open-minded and strong willed, she didn't know what the term meant, so she asked. Told that the removal of a woman's outer sexual organs was something that had been carried out in her culture – among many others – for hundreds of years as a way of preparing girls for adulthood and assuring their virginity, she was horrified.

"My first reaction was disbelief. I thought it was something that happened in [her mother's] time, that happened in Somalia. I didn't think it would be happening to girls who are my age, or in the UK," she says. "But then I found out that it was. All I can remember thinking was – why hasn't anyone tried to stop this before?"

Fahma is one of nine daughters in a Somali family that moved to Britain when she was seven. Now 17, she is part of a new generation of anti-FGM campaigners determined to make politicians sit up and listen, and finally end female genital mutilation. A trustee of the charity Integrate Bristol, which fights against FGM, she has now become the face of the Guardian's campaign to help end the practice. She puts it simply: "I want to help these girls who don't have a voice."

In a classroom in the City Academy Bristol – one of the few schools in the country running a dedicated anti-FGM project – she joined a spirited group of teenagers, many of them wearing headscarves, to practice a song composed for the UNs FGM zero-tolerance day on Thursday.

Calling themselves the #FDL or Female Defence League – with some of the cheekier older girls substituting the word "fanny" for "female" – the girls belted out a song that, as they put it, is "sayin' no to bullshit oppression". In a rapped section, one of them spits out her contempt of politicians, such as the education secretary, Michael Gove, whom she accuses of shirking the issue: "I'm sorry, Mr Micky, if you still don't get it / Then David Cameron oughta say, beat it, Gove, beat it!"

Confidence in the group has not always been so high. Lisa Zimmermann, a teacher who co-founded the organisation, says she became conscious of FGM when told that 11 of 12 girls in a group she was taking on a trip had undergone cutting.

At first the group was limited to four girls, who wrote anonymous poetry. Soon more joined, but when the girls made a film, Silent Scream, about FGM, it met fierce opposition and critics descended on the school.

"We were accused of making a porn film – people said we'd forced the girls to be in it and didn't want the film to be shown," says Zimmermann. After some of the girls' mothers met police to ask for support, the screening went ahead and since then the group has grown to more than 100 members, who have appeared on television, taught at other schools and on the UN's FGM zero-tolerance day will host Alison Saunders, the director of public prosecutions for England and Wales. "Proud does not begin to describe it," says Zimmermann.

Fahma says she is sick of people citing cultural tolerance as a reason not to tackle FGM, adding that many do not realise what a serious issue it is. "I definitely know people it has happened to: girls who have been taken home, or who have had it here. For some of these mothers it's a lot cheaper and easier to get them done here. I know these girls, and it just fuels my passion. I want to get their voices heard."

The consequences for her friends are severe. "FGM is a terrible thing. Not only physically […] I think people don't think about how traumatic that would be, how it will mess her up emotionally. People don't understand that this is something they live with every day of their lives, not just physically but emotionally."

She is directly appealing to Gove to write to every headteacher in the UK before the next"cutting season" – the summer holidays; when girls are cut in order to give them time to recover without it being noticed by teachers – asking them to train schools and parents about FGM.

"A girl's high school years are when she grows up, learns about her body, goes through puberty … School students need to know about FGM and they need to know that it's wrong," says Fahma.

Asked if she thinks the famously intractable Gove – who has faced down repeated attempts to get sex and relationship education made compulsory in the school curriculum, and has ignored requests for meetings from other FGM activists – will listen, she smiled. "We are not going to be quiet, we are not going to shut up," she says. "It has taken us this long just to get people talking about it, we don't care how long it takes."