"When you educate a woman, you educate a nation," says Kalunde, whose foster child was thrown out of her rural school and her home after becoming pregnant. "A woman is a mirror and spends much of her time with her children."

Every day across Tanzania, hundreds of schoolgirls become pregnant, bringing their learning to a halt. The taboo of young mothers returning to lessons is especially strong in the profoundly poor, drought-prone region of Shinyanga, where Kalunde lives, and rate of school pregnancies is rising.

"Education is an absolute priority in these regions," says Julitta Onabanjo, the UN Population Fund's representative in Tanzania. "It is really stark, in regions like Arusha and Kilimanjaro where education is valued, for example, women have an average of two to three births, but in places like Shinyanga there are seven births on average. It is still a widespread mentality in Africa that your family is your wealth."

Anita Masaki, project officer for the Forum for African Women Educationalists, the leading group among scores demanding an end to the exclusion of young mothers from schools, says: "If a woman is educated, she plans her life, she plans her family, she educates her children and lifts her descendants out of poverty."

Tanzania will mark 50 years of independence in December, and many, many more people will celebrate the anniversary than cheered the liberation: the population has rocketed from 6 million in 1961 to 45 million today. With 71% of the nation under the age of 30 – one of the youngest countries on Earth – the momentum for further growth is strong, with the UN predicting it will be the 13th most populous nation on the world in 2050, with 138 million people.

The Tanzanian government attributes the high fertility rate to "early and near-universal early marriage", low prevalence of contraception and the "low educational and cultural status of women". But national statistics show that education is fundamental to birthrate. A third of Tanzanians over 10 years old cannot read or write and those women with no education have an average of 6.9 babies. Women with a primary school education have 5.6 babies on average and those with secondary and higher education, just 3.2 babies. Furthermore, half of all women have given birth by 19 and 70% are married by 20.

"A lot of [the factors determining population change] are driven by education and what is invested in girls education in particular," says Onabanjo. "And it translates directly into higher incomes." That will be critical in a country where population growth has raced ahead of economic growth, leaving Tanzania the 172nd poorest nation in the world (out of 190), according the World Bank, with half the population living on less than $1.50 a day.

"The government really needs to slow this population growth down, or increase economic growth, which is very hard," says Onabanjo. "If Tanzania keeps going at 2.9% population growth a year, it is scary." She says those that argue a big population is good for a developing country have to explain how the extra schools, hospitals and jobs will be provided.

In the vast dusty fields and ramshackle towns of Shinyanga the problem is that sex education is minimal. Getting pregnant while at school is common and expulsion as a result is universal, the result of deep cultural disapproval and a pervasive prejudice that returning teenage mothers will be a bad influence. "It is the end of their educational life," says Masaki.

"Poverty is a key factor," says Mary Soko, Oxfam's education programme officer for Tanzania. "Parents force early marriages so they can escape the role of taking care of their daughter and her child."

"My father was very, very angry," says Ester, who started primary school aged nine and became pregnant in her last year, at 16. "He took me to the hospital. When they said I was pregnant he didn't say anything more. But the next morning my brothers took me to the house of the boy who fathered my baby, and we were made to marry."

Ester already has a second child with her husband, who makes a living using his bicycle to provide a taxi service. She says: "If I had been living somewhere else, I could have been someone else. I would like to have gone to secondary school and maybe studied how to make electricity."

Maria, an Aids orphan, became pregnant at 14: "My uncle chased me away and made me marry the boy. Maybe I could have become a nurse." The age of consent is 18 in Tanzania but, unlike boys, girls can be married at 14 with parental consent.

Other girls, like Kalunde's daughter, do not get married as the fathers often go into hiding to avoid the 30-year prison sentence prescribed for statutory rape.

Stigmatisation is intense. Elena, from a small village, dropped out after two years at secondary school when she became pregnant at 18. She says: "I was ashamed. After my baby was born, I went to live with my sister. My parents would have been insulted by their neighbours that they had educated a girl only for her to go and get pregnant."

Many young mothers have been seduced by older men with food or money, says Soko, during long journeys to school. Furthermore, education is often seen as a waste of time for girls. Over 60% of boys attend secondary school from age 15, 10% more than girls, of whom 5% will be pregnant by that age. Of those continuing their education after second level, men outnumber women two to one.

John Shija, at the Oxfam-supported Paralegal aid centre in Shinyanga, Tanzania's second most populous but third poorest region, says: "A great proportion of the poverty I see here is being caused by illiteracy. The parents are already intimidated by the schools, who say pregnant girls have expelled themselves, and there is self-stigmatisation by the girls." Raising someone's learning by one educational level can raise family income by 30%, research suggests.

Opposition to young mother re-starting school "comes from a patriarchal society," says Masaki. "They think unmarried pregnant girls have committed adultery, but forget that it takes two ... And they think an educated woman is not a good wife."

But, she says, there is no legal justification in any Tanzanian law for the expulsions. Her group has worked with Unicef and others to organise meetings with communities, teachers and to set up Tuseme (Let's discuss) groups for pupils. These change attitudes, but a national policy for the re-entry of girls has yet to appear, despite President Jakaya Kikwete saying in 2007 that "we must find the solution to this problem as soon as possible" and urging parents and community leaders to ensure girls completed school.

Neighbouring Zambia has introduced such a policy, and Masaki says: "The rate of [school] pregnancy has gone down, down, down, almost to zero." While 70% of teachers there opposed re-entry before the policy, 84% were in favour after.

Other nations in sub-Saharan Africa facing the same booming populations as Tanzania are having different levels of success in tackling the taboo of young mothers returning to school. The Democratic Republic of Congo, on track to have 149 million people in 2050 and be the world's 10th largest nation, is not tackling the issue, while Nigeria (349 million by 2050, 4th in world) and Ethiopia (145 million, 11th) are, says Masaki. All three have lower levels of literacy than Tanzania.

In Shinyanga, Teresa, who became a mother in primary school, where she liked studying mathematics, is not thinking about global population, the reaching of the landmark of 7 billion humans or even the future of Tanzania. But she has a simple message: "You must find ways so girls don't get caught up with boys, but learn to read instead."

Note: The names of the young mothers and their parents have been changed.