It was a muggy afternoon, and Nakesha Martin raised her voice to be heard over the rattle of the air conditioner. “Is that a high-risk behavior, or a low-risk behavior?” she shouted to the class.

“High-risk,” came a murmured response. Martin beamed.

“That’s right,” she said, passing out candy in the direction of the reply. “And what could we have done to be safer?”

Used a condom, someone suggested. Not had sex. There was a round of sheepish laughter.

Martin was on the outskirts of Tulsa, Oklahoma, where two dozen teens and several 12-year-olds had gathered as part of the city’s first serious push to offer every local teenager comprehensive lessons in sex education.

Over the next two hours, Martin doled out facts previous generations in Oklahoma, where sex education is not mandatory, were never taught. Yes, you can transmit HIV through oral sex. No, not through kissing. Later, the room crowded around a set of flashcards listing the steps for putting on a condom and tried to put them in order.

The class is part of a much larger initiative undertaken by Barack Obama’s administration in pursuit of a daunting goal: reducing teenage births and pregnancies in a country that has nearly always led the industrialized world in both. Here in Tulsa, several of the participants brought babies, who passed the time wiggling against the straps of their strollers.

But less than one year from now, this class and dozens like it in the region will probably vanish, swept away by the Trump administration’s decision to cut the entirety of the funding for the program, known simply as Teen Pregnancy Prevention.

The decision was announced with little warning in July.Eighty-one programs across the country, including Tulsa Youth Services, which funds Martin’s class, received identical notice that their grants would end on 30 June 2018 – and not, as planned, in 2020. The cuts add up to $213.6m.

The real cost, though, say grantees, will be borne by the 1.2 million teenagers the program was expected to serve.

“We have to teach this stuff, it’s important,” said LaKala Williams, 18, who went through a sex education course funded by the grant and is now involved with the education efforts. “We all want to lower teen birth rates, and so, why the cuts? Do you want these young kids to get pregnant? Is that what they want?”

Few places will suffer the loss as deeply as Oklahoma, where the teen birth rate is the second-highest in the country.

For two decades, the state has not dedicated a single dollar of its own budget toward preventing teenage pregnancy, relying instead on the federal government to fund 90% its efforts. Oklahoma has used much of that money to pay for some of the state’s first comprehensive sex education programs, which research has shown reduces early pregnancy and sexually transmitted infections. The cuts will shortchange those efforts by about $8m.

“The state of Oklahoma is not really in a position to lose that money,” said Heather Duvall, the director of programming for the Tulsa Campaign to Prevent Teen Pregnancy.

The timing makes the cuts particularly brutal, national advocates say, coinciding as it does with the greatest period of reduction in teenage pregnancy the US has ever experienced. Although the US rate has spent two decades in decline, the country’s teen birth ratio was until recently the highest in the industrialized world. In 2011, there were 57 teen births in the US for every 1,000 teenage girls, a rate roughly equal to Kyrgyzstan’s.

For the young women behind those figures, the consequences can be life altering. Williams has watched many of her pregnant peers become mired in a cycle of missed opportunities.

“They get embarrassed,” she said, and skip school, or they miss school because of stress, emotional or bodily, medical appointments, and time spent caring for their children. “They stop going to school, they go to online school, they stop going to online school,” she said. “Some of them don’t graduate, they don’t get to finish or further their education.” In the US, roughly half of all women who become teenage mothers never earn a high school diploma, and millions go on to live in poverty.

But the number of young women facing such a future is at a historic low. From 2010 to 2016, the teen birth rate nationally plummeted 41%. No other six-year period saw a decline even half that size. The shift took place right as the Obama administration was making a unprecedented investments in sex education and other programming proven to reduce teen pregnancy.

Now, the Trump administration has slashed that investment – all of it.

“It was shock, initially,” said Paulette Lassiter, recalling her reaction when she learned of the cuts. A window behind her overlooked the freeway to Oklahoma City, where the teen birth rate in some zip codes is twice the national average.

From this office, Lassiter manages a $1.2m grant that provides the city’s largest stream of funding for comprehensive sex education in its public schools. This region is the source of one in five Oklahoma teen births, yet the city for years made only sporadic efforts to fight against the trend.

This year, the number of Oklahoma City students receiving sex ed will top 3,000 and represent all grades from six through 12. But the year after that, Lassiter and the grant will be gone.

What would happen then?

Lassiter let out a loud, rueful laugh. “That’s the major question.”

Sitting across from Lassiter, Brittany Keck, who oversees the sex ed programs funded by the grant, brandished copies of several letters scrawled in round, teenaged handwriting. Sixth graders had written them after completing their first sex education class.

Keck read out loud. “I learned that if you are getting pressured to have sex, you should try to talk it out or find someone who wants to wait,” one read.

“It helped me a lot with planning for my future to go way better,” read another. “Now I can say no to things I feel I’m not ready for.”

One letter, from a sixth grader, captured the extra challenge posed because Oklahoma does not require health sciences: “I learned where the baby grows. I thought it grew in your belly.” In addition to new sex ed teachers, the grant paid for new lesson plans, because the old ones were developed in states where students had already been taught the basics of human anatomy.

Without the grant, Lassiter doubts Oklahoma City can meet its current goal of reducing teenage births one-third by 2020.

“I think what we’re going to see happen is, nationally – because these programs are national – the rates are going to go back up,” Keck said. “We’ve seen this national decline. I think we’re just going to see a national rise.”

The cuts are already taking a toll. In addition to slashing the funds for 81 programs like this, the Trump administration immediately terminated five related grants to major research groups.

One of those groups, the University of Michigan, was preparing to help Lassiter convert Oklahoma City’s public health clinics, which were designed for adults, into clinics equipped to offer STI testing and birth control to teenagers.

The whole process was supposed to be directed by university researchers who knew how to optimize health care for teenagers through years of trial and error. Then, just like that, their program was gone. A letter the federal government sent to the five groups said their work was no longer in line with government priorities.

The culprit behind the cuts is something of a mystery. One suspect is the Office of Management and Budget, which in May released a budget proposal gutting dozens of social programs. Another is the office of the assistant secretary for health, which indirectly oversees the Teen Pregnancy Prevention Program and is run by a Trump appointee, Valerie Huber, who has lobbied for federal funding of abstinence education.

“Exactly what is responsible sexual activity for adolescents?” Huber told the Guardian last year. “The science is clear that teens are healthier when they avoid all sexual activity.”

Her office did not respond to interview requests for this article.

Keck and others take issue with statements like Huber’s on abstinence-only education, which is not supported by the evidence as a way to prevent teen pregnancies or STIs; it merely delays sex without offering advice on safety. Besides, Oklahoma City’s classes do encourage abstinence; they are often students’ first introduction to the very concept.

Local advocates see little hope of containing the damage the cuts will bring.

“It will be hard to find a funding source to replicate that,” said David Grewe, the executive director of Youth Services of Tulsa. “$1.5m a year, for this community, is a huge investment. Where do you find $1.5m?”

He and others worry about what the future will hold, while Omare Jimmerson, the social services coordinator for Tulsa public schools, already has a guess.

Each year, dozens of teenage parents look to her office to coordinate transportation, healthcare, and childcare as they struggle to complete high school.

This is only the fourth year that all Tulsa public schools have offered sex education, meaning lots of those teens belong to a generation of students who never had a comprehensive lesson in sex ed.

“And so, this is what happens,” said Jimmerson.

Even with help, some students in her program will fail to finish school. Two of her enrollees this year are a mother-and-father duo who dropped out last year with plans, never completed, to obtain a GED (General Education Development diploma, certifying high-school level skills) online. Jimmerson is excited to welcome them back, but they underline the paradox she’s up against.

“It’s kind of like, ‘Oh yay, we have twenty girls enrolled! … Damn, we have twenty girls enrolled,’” Jimmerson said. “It’s starting at the bottom.”