Can we start rocking?" 17-year-old Jamie Harrison calls.
There are mumbles through the group. Then a pause and a scuffle while someone locates an iPod, speakers and a suitable playlist.
And then, to a soundtrack of Jessie J, the rocking begins. Jamie bends over the rubble of the building site, and picks up a large stone. He hefts this across to the younger boy next to him, who almost drops it. "Woah!" they both say. "That's why we have steel toe caps," Jamie says, grinning.
The second teenager passes the rock to the person next to him, who passes it along to the next person. And so on, down a line that stretches across the playground, until a sizeable pile of rocks has been collected at the outer limit of the school site.
The teenagers are building a toilet block. By hand. Almost 30 British students, aged between 12 and 18, are spending their summer holidays heaving rocks and layering cement in Kamoda, a remote village in the western Indian state of Rajasthan.
Building latrines, it turns out, is highly competitive: the teenagers all had to demonstrate prior social-action experience in order to be accepted for the job. Several are youth ambassadors for Save the Children; others have won Diana Awards - named for the late Diana, Princess of Wales - for charitable works. One 13-year-old boy spent a previous summer holiday organising a large-scale fundraising project across South African gyms.
Once accepted, the students are offered an all-expenses-paid trip to Rajasthan. Their places are funded by Virgin Atlantic, using the leftover foreign currency which holidaymakers put into charity envelopes on their flights home. Last year, more than #163;677,000 was raised on Virgin Atlantic planes, and donated to charity Free the Children.
Primarily, this money supports development in India and Africa. But it also pays for an outreach scheme for British secondary school students, who are invited to spend the summer working on Free the Children projects.
So, while Indian children in school-uniform shirts and pinafores look on, the British teenagers pile and cement rocks to an upbeat soundtrack. By the time they leave, they aim to have built the best part of a latrine block and a playground boundary wall.
Headteacher Prem Singh Rathore observes the teenagers as they work. "Right now, our girls don't drink water, because of the fear of peeing in the open," he says. "There's no privacy, and that's a bigger deal for girls than it is for boys.
"But for a bicycle to function well, we need to have both tyres running. That's why girls and boys need to be educated. It's the only way the village can improve. It's the only way the nation can improve."
No place like home
"Water break!" someone yells. "Water break, everyone." The teenagers down tools and rocks, and scramble over the monsoon-loosened mud for their water bottles.
By the infants' classroom, Dominique Newton takes off her builders' gloves, wipes sweat from her forehead and swigs deeply. "I'd never picked up an axe before," the West London student says, glancing into the classroom, where children sit three to a desk.
"To be honest, I've never really experienced poverty," she continues. "We hear about it on the news but, because it's so far away and it doesn't affect you, it doesn't really seem to be there. Here, because it's right in front of you, you can't ignore it. You just have to cope with it."
There is, however, something undeniably discomfiting about British schoolchildren spending their holidays as unskilled day labourers, so that their Indian counterparts are not condemned to exactly that kind of work.
This irony has not escaped the teenagers themselves. The water break over, 18-year-old Leanne Armitage from South London rolls up her sleeves and picks up a trowel.
"The build - how laborious it is, and the physical strain it gives - this is what people do as their daily jobs here," she says. "In England, there are so many opportunities. If you put your mind to it, you can do anything. In India, if someone from a village wants to be the next prime minister, it's highly unlikely that they can do that."
This is a significant realisation. In fact, the toilet block and the playground wall are, essentially, by-products of the experience: they will be completed after the group has gone home. The primary goal is something else.
And so, after a day of rocking and building, the teenagers are not left to their own electronic devices. Mobile phones are banned for the duration of the trip; parents are sent weekly emails, letting them know that their offspring are safe.
Instead, sitting on the open roof of their hotel, they are invited to imagine what it would be like to be part of a family living in an Indian village.
As beetles buzz around their heads and the electricity flickers and hums, they take on the role of local villagers, arguing against sending their children to school. "Would you willingly send your three-year-old daughter to school alone?" a blonde girl posits. "Or at the age of 5? I think not."
Next to her, a sandy-haired boy takes up the argument: "If all the kids go to school, who's going to do the work at home? And what's the point of learning about things, if you're never going to go outside your own village? Children should be at home."
He pauses, shifting awkwardly from foot to foot. "It's hard to argue for something you don't believe in. You have to think of your own argument and then go against it."
Tom Brookes, an outreach worker for Free the Children, is one of the trip leaders running the debate. "Yeah, it's very difficult to argue for something you don't agree with," he says. "But these are all the arguments that parents and families in rural India legitimately have to go through. Is it safe to send my daughter on a bus by herself? Is it more useful to have my son on the farm? As much as we hate to think about these decisions, they're all really valid."
Next to Brookes, Jamie is sitting on a mat, freshly showered after the day's rocking. The trip has, he says, given him insight into his own privilege: "In my high school, kids moan about not having enough free time. We say, 'Oh, the paint's flaking off the walls.' The girls say their make-up's not right. The boys say, 'Is my hair spiked enough?' or 'Do I have enough aftershave on?'
"Everything's on tap in Britain. And what have they got here? No running water in the school area. Nothing to wash their hands with. And have you seen the size of the classrooms? They try and fit 40 people into a little box. But they're happy to go to school."
Dominique sits on another mat nearby, swatting away swooping beetles. At 12, she is the youngest participant in the scheme. And, while she has made a concerted effort to keep up with the other builders, she is also aware that she is barely older than the Indian students she is helping.
"I've always wanted to know about how other people live," she says. "But the pupils here are practically my age, and their life is so much harder. Oh, it's heartbreaking to see people struggling just to survive.
"When we first came, I said, 'What's this?' They said, 'It's a classroom.' I just couldn't understand how it could be a classroom. If I compare my school to this one - it's just so unthinkable. I just think, how could a child honestly work or learn in such a hard environment?"
Passing the message on
The aim, says Brookes - and the reason that Free the Children is so careful with the selection process for the trip - is for the students to educate others at home about the realities of the developing world.
"Some of them come straight off the plane and back to school," he says. "It'll take a while to adjust. But I think they'll go back with a different way of looking at life.
"Young people have got a lot of empathy for other young people. So, right now, they're switched on about what they want to do at school, what charity work they want to do."
He is quick to point out that the project is not a recruitment drive: there is no pressure put on participants to campaign on behalf of Free the Children. Some will continue to work for the charity, or for other organisations based in the developing world; others may choose to volunteer in a local youth centre or an old people's home.
"If you're told to do something, you're going to do it half-heartedly," Leanne says. "But if you can choose something you're passionate about, the impact is going to be a lot more effective."
Next to her, Jamie nods. He already volunteers at a primary school in his home town of Wrexham. But this trip has focused his ambitions. "We're the future. We're the change," he says. "What's going to happen 10 years down the line if someone doesn't carry it on?"
Ultimately, he would like to train as a primary teacher. "I want to make children of the future respect and appreciate what they have," he says. He pauses. "I know primary children are young. But you can make them see that there's change to be done. I can hopefully inspire them, the way I'm being inspired now."