Facebook Twitter Pinterest Zehrah Hasan is a student barrister volunteering with the Vocalise scheme Photograph: Alicia Canter/Guardian

On a summer afternoon in a classroom in west London, eight individuals fiercely compete in a public debate on: “This house would legalise vigilantism in areas of high crime”. A speaker for the motion wryly explains that they needed vigilantism in his area because the police were so slow that a criminal had time to eat his lunch before law-enforcers arrived. The speaker knew his facts. He was the criminal finishing off his lunch.

The classroom is in HMP Wormwood Scrubs, and the proposers of the motion are four prisoners challenging four of the best university debaters in the country. And they won. The event was judged by a panel of QCs and avidly watched by guards, staff, fellow inmates and the student mentors who had trained them over the past three months.

I was one of the mentors. For almost a year now, I have been involved with Vocalise, the volunteering scheme administered by the Gray’s Inn education department which trains student barristers like myself to teach a 12-week debating skills programme in prisons across London. It has worked in Pentonville, YOI Feltham, Brixton and Holloway over the past five years, with plans to take on new prisons in 2016.

I have taught around 40 inmates in Wormwood Scrubs and 10 in Brixton alongside three other mentors. By the end of the course, these formidable buildings had witnessed some of the most fascinating stories I’ve been told, discussions I’ve ever had and people I’ve had the opportunity to meet.

Students in Brixton had particularly strong opinions on whether we should ban independent schools. Half the class believed they represented “elitist scum”, but other prisoners defended their purpose, saying although they couldn’t attend one themselves, they now put their own children through private education.

Even the most resistant prisoners became better listeners, reasoned thinkers and clearer communicators

For our first class in Wormwood Scrubs, we asked students to name their favourite film. Baddie said his was Law Abiding Citizen. Isaac went with Menace II Society.

I can see why some people could be sceptical about the benefits a weekly workshop on parliamentary-style debating has for offenders. But learning how to debate teaches more civic values than I could have anticipated.

Karen Bryan, pro-vice-chancellor and dean of the faculty of health and wellbeing at Sheffield Hallam University, investigates the prevalence of language difficulties in young offenders. Her research shows that in young offender institutions at least 60% have low language levels suggestive of some sort of learning disorder. She says that with the help of speech and language therapists, debating can provide an excellent way to build self-confidence and express disagreement or anger in a socially acceptable way. “Clearly, communication skills are important, but we don’t as a society actually value this,” says Bryan.

In our classes, even the most resistant students became better listeners, reasoned thinkers, clearer communicators and more keen to engage in teamwork, shortly after beginning the course. Inmates were often surprised by their own positive responses to lessons. As Ahmed, one of our students in Wormwood Scrubs, said: “I didn’t think I would enjoy public speaking, but I have had great fun. I always look forward to Vocalise on a Friday morning”.

And it goes without saying that these skills are transferable to post-prison life, helping with resettlement or new employment. “I will add this experience to my CV when released,” says Tony.

Alex Just and Florence Iveson, who founded Vocalise in 2010, modelled the project on a US scheme run by Sam Nelson, director of forensics at Cornell University. That programme drew its inspiration from Malcolm X, a strong advocate for the power of debating after his own life-changing experiences while serving time in the Norfolk prison colony. The civil rights activist wrote, “Once my feet got wet, I was gone on debating”. Watching the Vocalise students, it was clear that many of them were “gone on debating” too.

The prisoners in Brixton also enjoyed a victory in their final debate this summer, beating the Oxford Union. David Omar, a Brixton inmate for nine years, has attended the workshops since they started in 2010. He won the best debater award in last month’s competition, impressing everyone with his technique and charismatic delivery.

Jennifer Edward-Price, deputy education manager at Wormwood Scrubs, has seen at first hand inmates’ transformation. “I observed a prisoner who joined us in education who was rather subdued, very quiet and didn’t really interact with his peers,” she says. After attending a few sessions with Vocalise, his confidence had grown and he began demonstrating real leadership qualities and had a voice. My enduring memory of the event was how pleased the guys were after the debate and flashed their certificates, shouting “Miss, we’ve won!”.

I have seen this year how attentive teaching and community engagement can transform the experience of a criminal sentence into a rehabilitative and learning process. Criminal justice should be reparative, not punitive. While prison reform remains on the agenda, for now it is education departments and social outreach projects that bridge the gap between the community and the convicted. And they manage to change lives and perceptions on both sides of the classroom.

Prisoners’ names have been changed