Annie Hindley is bent double, head inside a large cardboard box, pulling out items of clothing, one by one. “X-large,” she says as she hands a chequered shirt to her friend Charlotte Lawrence before delving for more.

“X-large, x-large, medium.”

Charlotte is piling them up on the floor, carefully sorting by size. “What are these anyway?” she says. “Is that a shirt or a jacket?”

“I think they’re called lumber jackets,” says Annie as she dives for more.

Charlotte and Annie, Year 12 pupils from Grey Coat Hospital school for girls, London borough of Westminster, are getting their first taste of voluntary work, helping to sort clothing for the 1,500 homeless people who will stay in six shelters run by the charity Crisis Over Christmas. All around them are giant boxes overflowing with donated clothes waiting to be assessed, sorted and labelled to be taken to shelters the following week.

Groups of teenagers are busying themselves between rows of metal shelves, stuffing pillows into pillow cases, boxing up Barbour jackets and folding blankets. Huddles of boxes, plastic crates, racks of food, bottles, cans, cartons, paper, chairs, games and towels lie all around, ready for distribution to the shelters. In one aisle are trolleys of camp beds, in another bundles of TVs, hi-fi equipment, toasters, washing machines and fridges: all the stuff of domestic living that most of us take for granted.

“This is the first voluntary work I’ve done,” says Charlotte.

“But I will definitely do more. It’s so hands-on, you feel you’re really doing something to make a difference.”

Annie and Charlotte, both 16, have come a long way from their comfortable classrooms in Pimlico to be here, in a huge warehouse somewhere between Beckton and the Albert dock. For west London girls, this is way out east - beyond the blinking lights of Canary Wharf and the bleakness of London City airport. Just getting here shows commitment, especially after school on a cold, dark Thursday evening in December. But Annie and Charlotte are far from the only two who’ve made the journey; almost 70 eager students from schools across the capital have travelled for up to two hours to lend a helping hand.

Carlene McKissack and Nadine Russell-Henry are Year 12 pupils from St Martin-in-the-Fields high school, London borough of Lambeth, an hour-and-a-half’s worth of public transport away. “We wanted to do something to help,” says Carlene. “I’ve heard about the shelters on TV, and I wanted to learn more about what goes on.”

“Normally, we’d be at home watching TV,” says Nadine. “But I’m happier doing this.”

Like the others here, Carlene, Nadine, Annie and Charlotte have been spurred into action by Envision, a charity set up four years ago by a group of university graduates who wanted to encourage young people to engage with their communities; to persuade them they could make a difference to the world around them.

Tom Doust is one of the founders. “We wanted to flip the negativity in society and the media about young people not being interested in politics and their environments,” he says. “We wanted to find ways to get young people involved, because it’s rewarding and engaging, and because they do care. We wanted to make it cool.”

Starting with no money and working from a shed in the garden, Envision “jumped in the deep end”, targetting 16 to 18-year-olds. The method is simple: Envision sets up extra-curricular forums at schools, where small groups of students meet to talk about what they want to do. The organisation simply facilitates the process, putting the young volunteers in touch with charities or the local authority, and providing ideas and advice.

Projects range from raising money for overseas aid to holding assemblies on Aids; from organising a charity fashion show to putting up solar panels to power the school science block. “They often tend to be too ambitious at the start,” says Mr Doust. “Then they realise they’ve taken on too much and have to scale down. But failure is important - the process of reassessment can be very productive.”

The groups, which usually meet at lunchtimes, are facilitated not by teachers but two adult volunteers recruited by Envision to give up an hour or so a week. Many of these “Envision youth educators” (Eyes) work or live nearby. Helen Stear, for example, works in communications at the Home Office, close enough to Grey Coat to spend one lunch hour a week with Charlotte, Annie and the 30 or so other pupils who’ve joined this Envision group.

“It’s good to get out into the rest of the world sometimes and interact with young people,” says Ms Stear. “I can fit this into my normal day so it’s not too big a commitment, yet it’s still rewarding.”

One of the “biggest magnets” of the Envision model, says Mr Doust, is that children don’t see it as a school thing. “It helps that we’re not teachers.

We have no targets to hit, or tasks to tick off. Our reward is when they’ve been through a process, and say, ‘I didn’t realise I could do that’. It’s citizenship in action.”

Clearly, Envision has struck a vein of teenage activism and concern. Four years after starting with just five London schools, it now has 60 groups across the capital. The charity has six members of staff and is funded by charitable trusts, the Home Office and the Association of London Government. Only the handful of private schools involved pay anything, pound;500 each, while Envision is seeking Department for Education and Skills funding to sell the idea to partner organisations in other cities.

The pupils pick their own projects; homelessness is one of the most popular choices. Johanna Bradford, a pupil at Grey Coat, explains why. “We see homelessness all the time, we come across it every day on the way to school, so it’s part of our environment. We’re told not to give money, but we don’t know what we can do to help.”

For Annie it was seeing a homeless man being bullied on the Underground that pricked her conscience. “It’s horrible that people are so prejudiced,” she says. “People distance themselves. I wanted to find out more.”

Johanna, Annie and their friends are organising a soup kitchen at school to raise awareness among fellow pupils, and to raise money for a nearby shelter. The evening they spend helping Crisis gives them a chance to meet Envision members from other schools, and to see how a large charity goes about such a major operation.

They also get to meet “real” homeless people. In the far corner of the Beckton warehouse, Brian Hassanali and John Lewington are surrounded by a throng of 20 teenagers. “I started drinking at 13 because I was bullied at home and school,” says Mr Hassanali, a 38-year-old former oil company supervisor from Sheffield. “I started running away at 14, and first became homeless in 1987. My most recent period of homelessness started two years ago. I was at work on June 10 and on the streets on June 11.”

There’s an audible gasp from his audience. “It can happen like that,” says Mr Lewington, a 21-year-old from Liverpool. “I lost my job, then my flat, and ended up on the street. Once you’re there, your confidence dwindles and it’s difficult to get back.” Questions come firing in, and the pair pull no punches with their answers. “I was scared, lonely, petrified,” says Mr Hassanali. “All I wanted to do was escape into my own world. It’s not just physically cold, but emotionally cold.”

He explains how he used to sleep on the Strand “because it’s safer to be near people”; how he had his rib broken and lost a tooth in fights; about his ex-girlfriend who’s now in prison for murder; and why, if he hadn’t “got to the point where I wanted to care again”, he would be dead from drink.

But he also tells the young people here what a difference they can make.

“Generally, people treat you as if you’re not there,” he says. “It’s the people who make an effort - even just a kind word - who make such a difference.”

“What you’re doing here matters,” says Mr Lewington. “It shows that people do care, that you’re not alone on the streets.”

Kingsley Kehmo Jaffa, a 17-year-old from Queen’s Park community school, in the north London borough of Brent, has been listening hard. “It’s satisfying to know you can do something,” he says. “When he said how much difference it makes, it made me proud to be here.”

“At school we don’t learn anything like this,” says Robi Shah from South Hampstead high school, Camden. “We go out clubbing and see homeless people and we talk about it. Young people have been stereotyped as uncaring but that just allows us not to do anything. This brings it home; we’re actually seeing what’s happening and getting down and dirty.”

“Envision gives us trust and responsibility, it makes us want to do something,” says Robi’s friend Anabel Aihie de la Serna. “Now, I want to go into the shelters over Christmas and see how they work.”

“Yeah,” says Robi. “This isn’t enough. It makes me want to do more. This is just a start.”

Envision: www.envision.org.uk; tel: 020 7974 8440; email: vision@envision.org.uk. For information about Crisis Open Christmas, go to www.crisis.org.uk