When Bethesda, Md., high school student Jenna Kusek first saw where she'd be living for three weeks in Tanzania, she thought, "You've got to be kidding."

This hole in the ground is the toilet? A trickle of cold water from an elevated hose is the shower?

But Kusek soon gained a new perspective. The white stucco house she shared with other teen volunteers last summer was a mansion by local standards, and better than the concrete-block house they would spend their days building for a local teacher. A cold shower, she realized, was a luxury unavailable to the village kids. A year after the trip, tears come to her eyes when she talks about how guilty she began feeling about having access to any kind of shower.

"Compared to how people lived in the village, our housing was too good to be true," says Kusek, 18, a senior. "I knew before I went to Africa that I was blessed, but I had no idea how lucky I was. I can't believe now the things we once took for granted."

Kusek's experience is being repeated by an ever-growing number of American teens traveling all over the world, led by dozens of companies feeding an appetite not only for more exotic travel, but for travel with a purpose.

The latest player: National Geographic. This summer the nonprofit organization's new Student Expeditions arm will begin teaming teens with National Geographic Society archaeologists, photographers, scientists and writers to explore such things as the Inca Empire and the treasures of India, while also arranging for them to spend time working with impoverished children or participating in projects such as clearing forest trails.

Lynn Cutler, of National Geographic, said the organization commissioned a study and found that teens who can afford travel want purpose and personal development. Market growth is expected: This is the largest generation of young people in history, and a million children worldwide will turn 12 every year for the next decade. Even now, students make up 24 percent of all international travelers. They are traveling at earlier ages, going on more expensive trips and going to destinations farther from home than any previous generation, according to the Student & Youth Travel Association, a trade group.

Companies serving this emerging market produce trips with various degrees of work and play. One of Kusek's four weeks in Tanzania was spent on such activities as photographing exotic animals on safari in Ngorongoro Crater and hiking around Mount Kilimanjaro. The other three weeks of the Putney-sponsored trip, she lived and worked in the village of Miangarini, mixing concrete and stacking heavy concrete blocks. She valued both experiences equally.

Travel to distant lands for an extended period is expensive, and trips that include community service often include the added expense of project materials. Jenna Kusek's trip to Tanzania, for example, cost nearly $8,000, and money was an obstacle. In fact, her parents turned her down the first year she begged. By last spring her pleas became more sophisticated, says her mother, Jody, who takes frequent trips to Africa working on HIV/AIDS issues for the World Bank. Jenna told her parents that she needed to learn about the world outside Bethesda and reminded them they'd always told her she should make a difference in the world.

Jody Kusek says that she feels the money was well spent "helping to create a new person whose life now is more about awareness of others; she has been forever changed."

Jeffrey Shumlin, co-director of Vermont-based Putney Student Travel, says his goal is "to get young people at a vulnerable time and give them a perspective on the world and their own country and how they live."

He recently was sitting in a cafe in Rwanda and began chatting with a young American expat, Elizabeth Davis. She was shocked and excited to learn that he was co-owner of Putney: In 2001, as a high school junior, she had taken a Putney trip to Costa Rica, her first trip abroad. During the trip, she had helped build a water tower, volunteered at a local school and decided to spend her life working in the developing world.

After graduating from college in 2006, she headed to Rwanda. She's now with a grass-roots organization that educates orphans and street kids, and works with student leaders to foster healing of the psychological scars of the genocide there.

Said Davis in a recent telephone interview: "Like most of my friends, I grew up in the comfortable American bubble and really had no idea what life was like for people in the developing world. That high school trip played a huge role in making me the person I am today."