It's not every major exhibition at the Southbank Centre that would include a papier-maché Amy Winehouse and a ceramic pot in the shape of a policeman, with a CCTV camera for a head. But then the curators of Art by Offenders aren't your average expensively educated art crowd. When they shuffle into view for our afternoon rendezvous at Tate Modern, the huddled half-dozen move at a snail's pace and playfully push each other around like a gang of hoodies at a bus stop.

Essentially, though, this is what they are, a group of youth offenders aged between 13 and 16. Their role as curators of the show is part of their community sentence (for crimes ranging from theft to driving offences) and they seem like any group of teenagers off their turf and out of their depth. Most hadn't been inside a gallery until the previous week and they are now charged with the hanging of a show drawn from work made by prisoners.

“Today's visit is about getting the idea of a thematic hang in their heads,” says Eddie Otchere, a freelance curator whose brief is to give the teens a crash course in art history and curating. “We went to the National Gallery and had a look at chronological hangs and then to the National Portrait Gallery to look at social hangs, status and so on. None of them had been to either gallery before. The only reason they had been to the Tate Modern before is because of the slide [Carsten Höller's Turbine Hall exhibit], which they didn't see as a piece of art.”

The show is the annual exhibition organised by prison arts charity the Koestler Trust, which has been growing in stature in recent years, with Grayson Perry selecting last year's show. There has also been increased interest in this art from collectors, with the work reflecting the kind of intense “outsider art” that arises from a concentrated time under lock and key.

A week later I meet the group again at the trust's building, where the thousands of paintings, sculptures and drawings from which they have to choose are kept. The trust is based in the old governor's residence at HMP Wormwood Scrubs. “Some of the art we have here about prison life is pretty grim and I hope, in an old-fashioned way, that it may put them off ending up there,” says Tim Robertson, the Koestler Trust chief executive.

Otchere oversees as the group makes its selection, debating which drawing of the late rapper Tupac Shakur (of which there are many) is best and which paintings best reflect prison life. He is joined by the trust's arts assistant Dean Stalham, who is testament to the positive effect it can have on prisoners, having gone from convicted dealer in stolen artworks to up-and-coming playwright after entering work for the trust's annual award scheme.

The boys are easily distracted, but intense pieces of work draw them back into the process, with a set of savagely close-up ballpoint pen drawings of tangled hair catching their eye. Images of people are judged faster than abstract work, with one man's diptych of his daughter receiving particularly short shrift. “That's weak,” says one boy. “He shouldn't have wasted his time.” The boys all like a painting of a street from above, which somehow portrays both home and unease, although pictures of prison life are those that prompt most analysis. One, of a man waving his arms amid a group of fellow inmates, brings discussion of what he could be so angry about, while the seemingly endless parade of screaming faces are seen as prison's equivalent of Munch's The Scream, one of the few paintings some of the boys know by name. The crash course in art appreciation seems to have paid off. “You wouldn't find anyone on the streets doing art like the stuff we have seen in here,” says one vocal 16-year-old. “It has inspired me to do more of my own art and has made me think more about prison - like it is not nice to be in.”

The work selected includes everything from outstanding portraits of the Queen to a house made of matchsticks and pieces of prose beautifully carved into prison soap. It might not be to everyone's taste, but it is hard to view without being equally moved, disturbed and amazed.

Art by Offenders is at the Royal Festival Hall, Southbank Centre, London SE1, until Nov 8. See www.koestlertrust.org.uk for more on the Koestler Trust's work.

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