If you thought all teenagers were grunting, inarticulate lumps, tune in to BBC Two's The Speaker tonight.

Duncan Harrison and Irene Carter of St Mary Redcliffe and Temple School in Bristol

You will have your preconceptions challenged as the youngsters on the first series of The Speaker prove themselves equal to some pretty tough challenges.

Seven of the 160 14 to 18-year-olds invited to the televised auditions were from St Mary Redcliffe and Temple School. They were selected from more than 1,500 applicants after submitting one-minute video speeches on topics of their choice.

The Bristol teenagers, and their chosen subjects, were: Joe Lewis, 16, of Totterdown, who spoke about fish and chips; Hannah Dixon, 15, of Horfield, mosquito devices; Irene Carter, 17, of Southville, asylum seekers; Joe Hensey, 15, of Westbury Park, graffiti; Lewis Parker, 15, of Bedminster, traffic lights; Duncan Harrison, 15, of Horfield, global warming; Jack Chianese, 15, of Horfield, Coco Pops.

They all attended the regional auditions in Cardiff and repeated their talks in front of the judges – comic Jo Brand, basketball player John Amaechi and performance expert Jeremy Stockwell.

All the Bristol students have featured in early episodes of the show and two, Irene and Duncan, got through to the final 20, who would undertake televised tasks with high-profile mentors.

Contestants have already been seen by TV viewers at Speaker's Corner in Hyde Park, where they had to select an item from a bran tub and, after a minute to think, speak about it. Duncan's was an alarm clock and Irene's was some sunglasses. They also competed at Wembley Arena, where they had to go on stage and "sell" something of which they had been given no notice.

Princess Diana's brother, Earl Spencer, played host to the young speakers, who then had to act as tour guides at his Althorp estate. The number in the contest is now down to 10, and this week's episodes, tonight and tomorrow, will show the youngsters working with newsreader Kate Silverton at London Zoo on the theme of storytelling, and with Tony Blair's former spin doctor Alastair Campbell on the theme of persuasion.

Duncan said: "Some tasks were much better than others. You could not really prepare. It had to be on the spur of the moment. It is about challenging yourself, and connecting with the audience."

Irene said: "What I have learned from this whole experience is the difference between oratory and speaking from the heart. For a good speech, both are needed, but the passion is most important."

The semi-finals and final will be screened on Tuesday and Wednesday next week.

St Mary Redcliffe and Temple School head teacher Elisabeth Gilpin said she was delighted and proud at her students' success, especially as they were up against many youngsters from private schools who were practised in formal public speaking competitions.

"We don't see speaking as just the privilege of the elite few who are going to be in debating societies," she said. "We encourage debate in lessons and we are keen for students to challenge and discuss ideas."

Executive producer Kieron Collins said the show had reinvented the traditional speaking contests to make the television grade and helped debunk myths about the majority of teenagers.

"The speeches were only one part of the drama – and there's a lot more beside. There isn't really anything like The Speaker elsewhere on television. It's not just about public speaking – it's about communication and confidence in every form."

He is pleased at the way it has debunked the stereotypes about young people.

"Teenagers often get a really bad press. They're unfairly grouped together as a big, unruly band of hoodies who need to be fixed or cured. But the majority of teenagers aren't like that – and making the show has only confirmed that to me: they are bright, funny, honest, usually unafraid to speak their minds and sometimes even right."