Three years ago Josephine was due to get married. It didn't matter that she was only 13, or that her future husband was in his 40s with two other wives. She had been circumcised and was therefore a "woman", her father told her. So he negotiated a dowry: six cows, 10,000 Kenyan shillings (£120) and some bed coverings. Under Maasai custom, he was perfectly within his rights. But his little girl was horrified.

"I didn't even know that man; he was so old," she says. "I wanted to escape. Then I heard there was a safe place for girls like us."

Her chance came a few weeks later. She went to market with her brother, then slipped away when his back was turned, and ran. Two kilometres outside town she turned in the gates of Kajiado AIC girls' school and sprinted up the long driveway. The teachers listened as she told her story; they gave her a bed and a uniform and put her back in school. She hasn't returned home since.

Now Josephine is 16. She has swapped the striking Maasai beads and blood-red shawls for a badly frayed green uniform with gaping holes. She lives in a dormitory with other rescued girls and has just finished her primary education. "One day I will become a nurse," she says, "and maybe I can help save other girls too."

Later, Josephine's furious parents came stomping into the school looking for her. They were confronted with the formidable figure of Priscilla Nangurai, the school headmistress. Heated words were exchanged. Insults were thrown. But the parents went home on their own.

For the last six years, Kajiado AIC, 70 kilometres south of Nairobi in the heart of Maasai land, has been embarking on a brave project - the rescue of Maasai girls as young as nine from the dark tradition of forced marriage, and the gruesome genital mutilation that precedes it. Maasai men are allowed to give their daughters to decrepit old men in marriage; in return they receive a generous dowry of cattle and money. The practice is rooted in tradition but now it is "all about greed", according to school chairman Daniel ole Sapayia. "It's inhuman to give a child away at such a young age," he says.

Kajiado AIC offers a bed, safety and a chance for the girls to continue their education. The girls are spirited into the school by sympathetic teachers, government officials or police. Some are mentally and physically broken when they arrive.

Teacher Phyllis Leinah recalls one nine-year-old who had been forced into sex with a man in his 30s. "That was our worst case. The poor girl was scared stiff and damaged. She walked awkwardly and couldn't hold her urine, so we brought her to a gynaecologist," she says.

Priscilla Nangurai has become the nemesis of certain Maasai men and the patron saint of some of their daughters. "The girls are fighting their own culture," she says. "Their parents fear that if their girl goes to school, she may not get married - and they may not get the dowry."

A Maasai herself, Nangurai has made many enemies for rescuing the child brides. "In the beginning there was a lot of resistance. Traditionally, women do not address men - so if I stood up to speak at a meeting, the men would turn their backs in protest," she says.

Through promoting education, the school has found itself at the vanguard of the Maasai struggle to modernise. It uses talks and videos to warn girls about the mutilation of circumcision. Key community figures, such as Kajiado senior chief, John ole Sappur, have come on board. "Everyone else is getting educated; we fear we will be left behind," he says. Fewer than one Maasai child in three attends school, one of the lowest participation rates in Kenya.

But many Maasai insist that Nangurai and her staff are interfering in family affairs and should mind their own business.

"What's happening is wrong. The girls' fathers are right; their mothers will only misdirect them," says Nchue ole Matau, in Kajiado town. "Later in life, these children will be nobodies - adults without a past or a future."

Another father, Manangoi ole Kipeen, sent his six boys to school but none of his girls. "I sold my livestock to educate the boys, so I thought I would replenish it with the girls," he says.

Removing a minor from her parents is probably illegal and the school knows it. But the government has turned a blind eye to the law and the police have intervened to protect the children and the school when necessary.

Agnes Nailantei was 11 when her sister whispered that she was to be given away. The next day, she got up at 5 a.m. and sneaked past her father to a waiting car outside. A sympathetic teacher drove her to Kajiado.

Now Agnes is 14 and prospering in grade seven. She has never returned home, although her mother visits from time to time. What is her opinion of married life now? "I would have been very unhappy, with many children and not enough food," she says.

Many mothers also secretly help with the rescues, risking severe injury in a culture where wife-beating is acceptable. In Mile 46, a village deep in the bush and miles from Kajiado, Konina Tarayia explains why she left her husband. He had married off her first three daughters, sometimes drinking the dowry money and returning home with only a kilo of sugar. So when 10-year-old Evelyne was betrothed to a 50year-old man, she smuggled her off to Kajiado.

"I wanted a better life for my daughter than I had," she says in her house, a low-roofed, dark hut fashioned from sticks, mud and cow dung. "Now I am very happy Evelyne is in school. Maybe she will get a good job and her father will realise he has thrown away the lives of his other girls."

Evelyne still has sharp memories of the day an old woman neighbour came around with a razor-blade to circumcise her before marriage. "It was very painful, but there was no alternative. I was recovering for a month with no medicine, just milk to wash my wounds," she said quietly. She is still angry with her father: "I blame him and I don't want to meet him."

Now Tarayia is trying to persuade women in her village to abandon circumcision. "I have told these ladies I will take a scissors and give just a little pinch, to fool the men," she says, smiling. But change is slow to come and many women have spurned her approaches. "Yes, our culture is beautiful and we appreciate it. But nowadays we see some of it is bringing more harm than good," she adds.

Last week marked the end of term at Kajiado AIC and the start of the longest holiday of the Kenyan school year. It is also the high season for celebrations, including marriages and circumcisions. Some of the rescued girls will stay in a new hostel on the school grounds; those at peace with their parents will go home.

Nangurai will spend the holidays looking for money. Abandoned by their parents, the rescued girls, of which there were 34 last year, depend on the generosity of well-wishers for the 17,000-shilling (£200) school fees.

Having completed grade eight, Josephine Tayiani wants to go on to secondary school, maybe in Nairobi. But there's no money yet. "I'm keeping my fingers crossed someone will come. We live by the grace of God," says Nangurai.

On Tuesday morning last, more than 600 girls packed into a small hall. When the academic awards had been handed out, the deputy head, Nicolas Muniu, spoke. "These holidays will be very long. Please avoid overnight meetings with `cousins' during the ceremonies. You are too young to become mothers."

He had a special word for the older girls: "Take care of yourselves. If you want to get married, fine. But if you don't, remember we are here for you."