With each descent, Daysha Matthews grimaces. She’s moving only a matter of inches, down and up, squatting and standing. But the way Matthews is gritting her teeth, she may as well be climbing Mount Everest.

Daysha, 15, is standing in her high school gym, at Thurgood Marshall Academy in Anacostia, surrounded by classmates who are also doing various exercises: sprints, lunges and the dreaded squat-push-up-jump combo known as a burpee.

Among them circulate five coaches from District Crossfit, a gym in Northwest Washington. On this fall day, they are visiting the public charter school to teach kids about fitness and nutrition.

About 20 of Thurgood Marshall’s 400 students have volunteered for the workout. That’s a drop in the bucket among the more than 70,000 public students in the District, many of whom deal with obesity, inactivity and other health risks.

“Just a few more,” says one of the coaches, Noah Gabriel-Landis, as Daysha pumps out another half-squat. “I know you’re hurting, I know you’re tired, but we’re almost there,” he says sympathetically. “Get as low as you can.”

Gabriel-Landis doesn’t harp on the usual mandates he reserves for adult clients: knees out, hips below parallel. Today, “below parallel” is not the point. Today, the goal is to get these kids moving.

As many gym teachers, parents, pediatricians and even government officials will attest, getting kids moving can be an uphill battle.

Nearly a third of American children are overweight or obese, according to the Centers for Disease Control and Prevention.

In the District, 19 percent of high-school-age boys and 16 percent of high-school-age girls are obese. The ward with the highest rate of obesity is Ward 8, where Thurgood Marshall Academy is located and where many of its students live.

Childhood obesity has been classified as an epidemic, prompting the creation of a presidential task force and public awareness campaign. The blame has been spread far and wide, on the fast-food industry, big agriculture, and, more recently, parents.

But at Thurgood Marshall, it’s not about placing blame — it’s about getting kids to take responsibility .

“One push-up is hard,” says Annie Scogin, the teacher who brought the coaches here. “One squat is hard. Life is hard. I hope that’s something they get out of this — that if it hurts, that’s okay. At the end of the workout, I want to see smiles. Tired, panting smiles.”

For residents of Ward 8, the problem has been access. Gyms are expensive, and fresh food is hard to come by.

According to D.C. Hunger, only three of the city’s 43 full-service grocery stores are located in Ward 8. Ward 3, far more affluent, has 11. Just three of the city’s 30 farmers’ markets are located east of the Anacostia River.

To address the access issue, Thurgood Marshall built a new gymnasium, which it shares with neighboring Savoy Elementary School, and began offering a variety of after-school athletics.

The school also provides free breakfast and lunch every day, participates in the D.C. Farm to School Network, which offers locally grown produce at a discounted price to students, and boasts the District’s largest organic teaching garden.

But, Scogin says, access wasn’t an automatic fix.

“I still see so many of my students come to school with a big bottle of blue soda and potato chips they bought at the corner store,” says Scogin. “I say, ‘What is this?’ And they say, ‘That’s my breakfast, Ms. Scogin.’ ”

Still, Scogin says her students are curious.

“They want to know how they can avoid diabetes, be able to do a pushup, or even just figure out what the heck hummus is.”

Scogin says that for a long time, she dreamed of bringing in someone from outside to talk to students about health.

But trainers at gyms she has belonged to “never felt like the right fit.”

“I’d never want my kids to think exercise and eating well is about being skinny or having big muscles. It’s about a lifestyle.”

After dealing with chronic pain from running, Scogin, the school’s track coach, joined District Crossfit this year to build all-around strength.

“As soon as I met these coaches, I knew they were it. For them, fitness is a part of life. I want that for all my kids,” she says.

“It hurts,” says Matthews, panting. She’s done each exercise four times now, and she’s tired.

Everyone is.

Some students are leaning against a wall. One girl is lying down.

“Two more minutes,” calls out Andrew Killion, one of the coaches and the owner of District Crossfit.

The entire workout is eight minutes long.

When Killion finally calls time, the students collapse in a heap and gather for the nutrition portion of the lesson: Protein, fruits and vegetables and other nutrient-rich carbohydrates, and healthy fats such as nuts and olives are good.

“Things that come in plastic wrappers from vending machines” should be limited, Gabriel-Landis says, careful not to use the word “bad.”

Even after they’re dismissed, the students linger. They snack on grapes, carrots and guacamole — “green stuff” that would later become the talk of the school, according to Scogin — and chat with the coaches.

Some ask for help with their squatting form; others ask when the coaches will come back. They are audibly disappointed to hear it was a one-off event.

Scogin interrupts the “awws” to remind the students that everything they learned can be done at home. She knows her advice will likely be processed like an optional homework assignment.

“This won’t change their lives today,” she says. “But hopefully we’ve planted a seed.”

Daysha, for one, is inspired.

“I feel the burn in my legs,” she says. “The challenge was unexpected. But I feel pumped.”

She’s still panting, but now, she’s also smiling.