The aroma of canteen bolognese sauce and curry wafts across the chatter-filled corridor and into the school assembly hall, where half a dozen actors are rehearsing their lines and moves. The centre of the hall suddenly becomes a parade ground, one corner a military prison. Rows of chairs appear and the space changes into a classroom where a new pupil is being ridiculed.

Despite the venue, this is no teenage make-and-mend production but the latest groundbreaking play being staged by National Theatre Wales: an examination of the life and times of the school's most famous former pupil, Bradley Manning.

Manning, the US soldier accused of betraying his country by divulging secrets via WikiLeaks, spent his formative years in the far south-west of Wales with his Welsh mother, and attended the Tasker Milward comprehensive in Haverfordwest.

So when National Theatre Wales, which produced the Michael Sheen play The Passion, decided to stage a version of Manning's life, there seemed no better place to rehearse and open the play than the corridors and hall that he knew so well.

The play, The Radicalisation of Bradley Manning, was written by Tim Price, who was taken aback when he read in the Guardian that the man at the centre of the WikiLeaks saga had Welsh roots.

"I'm from Aberdare in the valleys and I was bowled over that at the heart of this global story was this person that I had things in common with," said Price. "We've been through the same education system, he's played rugby in the rain, he's struggled with Welsh language mutations and now he is at the heart of this information war."

Price began trying to find out everything he could about Manning, speaking to friends, old schoolmates and teachers. The Labour MP Ann Clwyd, who pressed the government to acknowledge Manning's British roots, helped Price, as did the UK Friends of Bradley Manning. "There's enough anti-Bradley information out there saying he's a traitor, a bed-wetting mentalist," said Price. "I felt there was room to explore Bradley's politics, his upbringing."

Manning is the son of a US serviceman, Brian, and a Haverfordwest woman, Susan. He is a US citizen but also, under the British Nationality Act 1981, British by descent. He attended Tasker Milward from the age of 12 to 16. "A recurring theme was that he was far more politically astute and aware of the world than any of his classmates," said Price. "His friends say that while they were talking about girls, he was talking about Iraq. He was plugged into global politics."

The play is to be performed at Tasker Milward and also at schools in Cardiff and Flintshire in north Wales. It is not aimed specifically at students, although it is hoped some of them will come. Price's creation will also be beamed around the world via the internet along with links that will allow viewers to look at some of the source material while they watch.

Price arrived with the cast for this rehearsal at Haverfordwest on a minibus from Cardiff. It was the first time he had been in the school. "It's unnerving to think this was Bradley's domain. Every day for four years this is where he hung out with friends, this is where he grew up. To be putting on a story about him in this school is wonderful but it also feels quite complex."

The writer was delighted when the cast jumped out of the bus to sniff the air and identify "school pizza". The rehearsal is punctuated by shrill school bells – the same ones that Manning would have heard. Later in the day a group of drama students break away from their work on Brecht and watch the rehearsal. One of them knows a lot about Manning because a friend is related to him.

The director John McGrath said he hoped the cast would "dig into the place". "A third of the scenes are set in this school so by rehearsing scenes here there is a huge amount of information for them to draw on."

McGrath believes the story of Manning, who was held in solitary confinement for almost a year, is important in itself but also because of the light it casts on what people in their early 20s – Manning is 24 – are doing to change the world. "I think maybe this is the first time since the 60s when people in their 20s are doing the political running, from the students' protests to Occupy to the Arab spring."

The six actors who play 30-odd parts (all of them, including the two women, standing in as Manning at some point to chime with the I Am Bradley Manning campaign, in which supporters post images of themselves holding a placard repeating that slogan) are all in their early 20s, partly as a reminder of his youth.

Both Price and McGrath are keen to emphasise that this is not a campaign piece or documentary theatre but a drama, one version of a life story that may not be fully told for years.

Price, who hears during the day that he has been nominated for an Olivier award for his play Salt, Root and Roe, has written to Manning telling him about the play and hopes that one day he may be able to see it. He is convinced Manning is a hero. "He's a 24-year-old guy brought up in Wales, he's also a symbol and represents where the west can go so badly wrong."