On a recent Tuesday, the airy rooftop greenhouse at P.S. 333 on the Upper West Side of Manhattan was a hive of purposeful activity as students massaged olive oil into kale and ground spices for a salad they were preparing to serve at lunch.

“Could we spread this out and make it look edible,” Yadira Garcia urged, as the children drizzled balsamic dressing over a mass of spiralized zucchini and garnished the tray with cherry tomatoes plucked from a sprawling vine growing a few feet away.

Ms. Garcia, a graduate of the Natural Gourmet Institute in Manhattan, is one of a growing number of professional chefs assigned to the city’s schools. As part of a program called Wellness in the Schools (WITS), she is spending three years at P.S. 333, teaching cooking and nutrition classes to children and organizing educational events for their parents after school hours.

The program, which aims to teach public school students about healthful eating and fitness, partners with chefs like Ms. Garcia and Bill Telepan of the restaurant Oceana to develop nutritious recipes children are excited about trying. It’s a modern take on the home ec class. The goal is not to try to make kids eat food that they won’t like but to make things they already like healthier, and to instill an interest in cooking fresh, healthy food that carries over to the way the children eat at home.

WITS, now a national nonprofit reaching 50,000 children in four states, is the brainchild of Nancy Easton, who spent 25 years as a teacher, mentor and administrator in the New York school system. Ms. Easton was appalled to see overweight students unable to walk up a flight of stairs without stopping to catch their breath at the school she directed on the Lower East Side in the 1990s. That was long before the childhood obesity epidemic became big news. Of public school children in New York City, 40 percent are either overweight or obese, which is higher than the state average of 32 percent.

“Schools are examples for reading, writing and arithmetic, examples for how to behave,” Ms. Easton said. “They need to become examples for healthy living.”

She started by cooking wholesome meals for a handful of her own students. Gradually, a formal program to combat obesity took shape and eventually spread to more than 100 schools (87 of which are in New York). WITS recently added a program called Coach for Kids, in which trained fitness professionals lead children in activities at recess and after school. Ms. Easton says her goal is to be in all of the city’s elementary schools within 10 years.

WITS, which is based in Harlem, prioritizes working in underprivileged neighborhoods. The program is financed through grants and private donations. Schools contribute part of the cost from their supplementary budget on a sliding scale calibrated to the income level of the students’ families.

WITS introduced its alternative menu in partnership with the city’s Department of Education. For many lower-income children, school lunch is the only cooked meal of the day, supplying them with over half of their daily nutrition. Although the WITS program does not depend on government funding, some school nutrition advocates are concerned about efforts to weaken or defund federal initiatives, including a bill introduced by Representative Steve King, Republican of Iowa, in January, which would repeal nutrition standards for the national school lunch and breakfast programs.

Claire Lowenstein, the principal at P.S. 333, says the WITS program has been an excellent investment. “More of our students are eating the school lunch and fewer are bringing their food from home than ever before,” she said.

Gone is the usual kid-bait like fried mozzarella sticks and chicken fingers. In their place are a daily salad bar, wraps and main dishes like the Caribbean specialty chicken guisado and vegetarian dumplings, created whenever possible with unprocessed, seasonal and locally farmed ingredients.

Judging from the children’s plates in the lunchroom heaped with salad and bowls of vegetarian chili, the healthy new fare is a success.

“It’s insane how much they love salads,” said Mr. Telepan, who was at the school recently to help with the meal for a wellness event. “It is a known fact that it takes kids between eight and 15 times of trying something before they like it,” he said, adding, “It can take adults 30 times.”

Mr. Telepan, who joined the WITS team in 2008, recalled how students at a school in the Bronx took instantly to the celery root soup and baked rutabaga fries with cranberry relish he cooked for them, although none had ever tried these vegetables before.

“You’ll rarely find a kid who won’t eat something that they themselves have made,” Ms. Garcia said. “They turn into the best salespeople. They are like ‘you’ve got to try this,’ with their friends. It’s contagious!” Kale chips were a recent hit, and so was the black bean brownie, in which beans, oats and honey stand in for flour and processed sugar. “Sounds kind of gross,” one of the young salad makers said, but it turned out to be creamy and delicious.

Like other WITS chefs, Ms. Garcia helps train the kitchen workers to prepare healthy food. That can be a challenge. Many school kitchens are lacking in even basic equipment, and cafeteria staff members are not accustomed to cooking from scratch with fresh ingredients. There are also financial constraints: Nutritious meals can be hard to create on food budgets that can be as low as a dollar a day per student served.

Some argue that processed foods offers inexpensive calories, and that the fresh fruits and vegetables championed by WITS are beyond the budget of many low-income families. Ms. Garcia acknowledges that quality ingredients don’t always come cheaply. But she points to initiatives like the city’s Health Bucks program which helps home cooks stretch food stamp dollars by paying bonuses for healthy purchases.

Ultimately, she said, “we either pay the farmer or the pharmacist.” In the neighborhood of the South Bronx where Ms. Garcia grew up, there were high rates of preventable and reversible diseases like high blood pressure, high cholesterol and Type 2 diabetes — all illnesses that research links to poor diet. Her own parents have cardiovascular conditions that she attributes to their diets.

While adults like to talk about the health benefits of eating well, for the kids the reason is often simply that it tastes great.

Ike Shaw, age 10, tried vegetable sushi for the first time in Ms. Garcia’s food lab and became a big fan. He now makes it for his family at least twice a month.

“I was like, how can something be so healthy and so good at the same time,” Ike said as he chopped carrots, cucumbers and orange bell peppers in his family kitchen together with his 11-year-old sister, Sai.

She has introduced the family to new salad dressings and soups, which she also learned to make at the food lab. Their mother, Amy Shaw, a lawyer, says that whatever they try in Ms. Garcia’s class, they insist on recreating at home — and she stays out of the kitchen.

Ms. Shaw admits that she is not much of a cook, but says her children’s enthusiasm has been infectious. Cooking is now a family affair that has brought them closer. “I love that they are so passionate about it,” Ms. Shaw said. “Seeing how confident they are around cooking helps me relax around it too.”

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Fudgey Black Bean Brownies

by Chef Yadira Garcia, adapted from Natural Gourmet Institute

Ingredients

1½ cups black beans (1 15-oz. can, drained and rinsed very well)

2 tablespoons cocoa powder

½ cup quick oats

¼ teaspoon salt

⅓ cup pure maple syrup or honey

½ tablespoon stevia

¼ cup coconut oil, at room temperature (liquid)

2 teaspoons pure vanilla extract

½ teaspoon baking powder

½ cup to ⅔ cup organic semisweet chocolate chips (more if desired)

Preparation

1. Heat oven to 350 degrees Fahrenheit. Grease an 8-inch-by-8-inch pan.

2. Combine all ingredients except chocolate chips in food processor and blend until completely smooth.

3. Stir in the chips and pour into prepared pan. (Optional: sprinkle additional chocolate chips on top)

4. Bake 15-18 minutes, then remove from the oven and let cool for at least 15 minutes. Cut and serve.