Ten students are left alone in a classroom. They're alone because Miss Tomlinson has gone with Jamal, whom she’s reported to the police under Prevent – a controversial government policy which aims stop people becoming radicalised. Her absence unleashes a maelstrom of emotions and issues, from terrorism and racism to cyberbullying and family breakdown, in a very powerful piece from this accomplished young company.

Playwright Anders Lustgarden wrote Extremism for the National Theatre’s Connections scheme, so it's designed for young people to perform and is perfect for Shadow Syndicate’s superb ensemble playing. It's traditional at this point to say that Shadow Syndicate bring a play to the Buxton Fringe every year, and that it's always excellent quality. But if that's become a cliché, it's because it is true – and for their tenth year at the Fringe, they have chosen a thought-provoking and deeply disturbing piece.

The opening scene is wonderfully performed. There are multiple overlapping strands of dialogue, as the topics bounce around from speculation over what has happened to Jamal and what he might have done, through classic playground teasing, to Evan’s constant moaning about being hungry (nicely judged comic relief from Conrad Simpson). Themes emerge around what it means to be in the group, or to be an outsider; Alissia Di-Cosimo as Suhayla deftly handles some exposition-heavy speeches, exuding intellectual authority and calm under pressure. She defends herself against Darren, with all his assumptions and presumptions about speaking on behalf of “our own country”, and Alex Doran successfully epitomizes the Daily Mail in school uniform.

As the pressure to conform becomes sinister, the explosion into violence is raw and frightening. Isabel Milner as Melina is utterly chilling as she emerges as the ringleader, and I liked Will Mulcahy’s bluff Chris, a bad boy who nevertheless knows the difference between right and wrong and is deeply uncomfortable with unfairness. But, of course, it is entirely invidious to pick out individual actors in a production that is characterised by superb ensemble playing – particularly in those early scenes, where our attention is diffused across different points of interest and the emphasis is switching rapidly between them.

As it progresses, the play coalesces to a single focus, and some later scenes can get a little static with the actors standing in an arc. The dialogue loses some of its punch in its deliberate pursuit of emotional effect, and the pacing in key places allows attention to wander.

But overall, Shadow Syndicate’s Extremism is wonderful theatre – not just good for youth theatre. It's reminiscent of The Lord of the Flies in its depiction of how young people (and maybe not just young people) can turn inwards when inspired by a charismatic leader, and that it can take moral strength and physical courage to stand up to that effect. This play simply couldn’t be done as successfully by an older group, so do get along and see one of the best shows of the Buxton Fringe.