We try haranguing. We try pleading. We try bribing. But we still can’t get children to eat their vegetables.

Now schools, nutritionists and behavioral scientists are putting science to work to figure out how to get children to reach for a carrot instead of a candy bar.

And they’re seeing gains from strategies such as changing the placement of vegetables on the food line, giving vegetable dishes names that sound more appealing and hiring chefs to redesign cafeteria meals.

There’s urgency behind these efforts. The nation’s childhood-obesity rates have tripled since the 1970s, and legislators are clashing over the effectiveness of recent federal rules mandating healthier fare in schools. While it seems the regulations are getting more children to make better choices, there hasn’t been a measurable health impact just yet.

Here is a closer look at five innovative interventions that researchers have found can make a difference in schools:

In April, the Howard County, Md., school system opened all-you-can-eat salad bars at three elementary schools and moved them to the front of the cafeteria. Even before children entered the lunch line, they could see an array of five different fruits and five different vegetables a day, about 80% of it locally sourced. After the change, the number of students who buy school lunches increased.

“It’s all about selection,” says Brian Ralph, director of food and nutrition services for the system, which serves 55,000 students daily. “Hopefully, when students see something they’ve never tried before—like kale or romaine lettuce—they’ll come home and encourage their siblings to try it, too,” Mr. Ralph says.

Location matters, says a recent study by Arizona State University, which found that when a salad bar in a school cafeteria is made more visible, vegetable consumption is five times as high as before.

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Another strategy attacks the idea of early placement from a different angle—by giving children vegetables as snacks when they’re at their hungriest. Traci Mann, a psychologist at the University of Minnesota, performed a study where she offered children cups of baby carrots before getting lunch. About half the students at the Richfield, Minn., elementary schools chowed down on the carrots—a significant improvement over the typical 10% who chose carrots at lunch.

“By giving the kids carrots and nothing else, we are catching them when they are most hungry and putting vegetables in a contest they’re likely to win,” says Ms. Mann.

Stealth nutrition can be simple. Slicing up fresh fruit (children with braces would rather opt out than wrestle with an apple) and swapping out industrial gray tubs for colorful bowls have successfully changed children’s habits. Before the tweaks, at Idaho’s South Fork Elementary School, 30% of students who chose fruit tossed it out; today, that number has dropped by half, says Heidi Kessler, director of the Cornell Center for Behavioral Economics’ Smarter Lunchrooms movement, which aims to give schools evidence-based tools to improve children’s eating behavior.

Cool names (such as X-Ray Carrots or Turbo Tomatoes) and attractive signage have also been shown to entice more elementary-school students to eat vegetables, according to Cornell researchers. Another Cornell study suggests that children were more likely to choose an apple with an Elmo sticker than when there was no icon—so branding might be used to benefit nourishing fare and not just sugary treats. Other marketing suggestions from Cornell and other sources: getting the principal to serve a new dish, and bringing in food trucks.

The idea is to grab students’ interest. “If you are offering a new entree, you can’t just put it out there. You have to make a splash, because we are all creatures of habit,” says Ms. Kessler, adding that the approach has buy-in from lunchroom staff. “They like it because it’s something they have control over. They don’t have to write a grant or ask their school board for permission…but show them how to prepare broccoli so kids will eat it and they’re all ears.”

What ends up in stomachs vs. the trash? To get a more accurate snapshot, Rush University Medical Center in Chicago and Canyon Ranch Institute in Tucson have collaborated to design and test a program that tracks what students are actually consuming.

The pilot, launched in 2014, monitored children at two Chicago elementary schools with primarily low-income populations. Cafeteria workers scanned each student’s ID card and used a touch screen to document each item chosen. Then, researchers recorded what percentage of food was left untouched. Waste averaged 35% for entrees, 73% for produce and 30% for milk, according to the findings, appearing this month in the Journal of Nutrition Education and Behavior.

Such data helped the lunchroom staff identify the most popular items (vegetarian lasagna) and ditch the losers (chicken cordon bleu). What made this intervention even more effective is that parents received a customized report card at the end of each week, giving them a look at what their child consumed and a summary of its calorie content and vegetable servings. They could then compensate for any nutritional deficits at home.

While it didn’t get students to eat more vegetables, the system could help teachers and parents in any school monitor students’ food choices and measure any strategy to improve them. “Most Chicago school parents are generally unaware of what their children actually eat at school or how that fits into their overall diet,” says Brad Appelhans, associate professor of preventive medicine at Rush and a principal investigator of the study. “Now they can have a much clearer picture.”

Bringing professional chefs into cafeterias might seem like an indulgence. But such collaborations are beneficial.

A study by researchers at the Harvard T.H. Chan School of Public Health that appeared in JAMA Pediatrics in March 2015 concluded that both short and long-term partnerships between chefs and school cafeterias brought a 30% bump in produce selection and consumption. (In some cases, schools actually bring a chef on board to cook meals; in others, the chefs simply design menus.)

“For most kids, the introduction to vegetables is so terrible, I fear we’ve turned them off altogether,” says Ms. Mann of the University of Minnesota. “Most adults I know don’t want to eat raw cauliflower, so why would kids? But roast it and it can be delicious.”

More than 18% of responding schools in a 2016 School Nutrition Association survey have chef partnerships (up from 12% in 2014) to enhance menus and retool favorites. For instance, they offer nachos with low-fat cheese instead of canned yellow glop.

A child’s first encounter with a new vegetable is a crucial moment. Nutrition education can demystify unfamiliar vegetables and break down children’s resistance to trying them.

For example, Boulder Valley School District in Colorado takes students on field trips to farms, where they see exotic vegetables, such as purple cauliflower and watermelon radishes, being grown for their school meals. Also popular: Rainbow Days (when children are encouraged to try at least three colors of fresh produce from the salad bar) and Harvest of the Month side dishes, complete with collector cards touting fun facts instead of batting averages.

Similarly, experts at Children’s Mercy Hospital in Kansas City have developed a curriculum that emphasize a healthy relationship with food. In their Food Scientist program, 5- to 7-year-olds focus on their senses to experience new fruits and greens. How does it smell? Is it bumpy or smooth? Does it crunch or slurp on the tongue? On average, even the most picky eaters learn to accept new fruits and veggies within six months.

While it’s important to make the connection between fresh ingredients and wellness, the focus is on exploration, not preaching. “It’s all about facts,” says Brooke Sweeney, medical director of the Center for Children’s Healthy Lifestyles and Nutrition. “We’ve taken out the emotion, so it’s not about ‘good food’ and ‘bad food.’ “

In New York City, the Wellness in the Schools Program instructs about 30,000 public-school students on how to shop and read labels, and conducts hands-on cooking demonstrations with fresh produce, which not only nurture culinary skills but teach habits that will last a lifetime. The initiative has now expanded to Kentucky and Florida.

Experts say that even more important than funding is starting early. “To third-graders, who have been in school since the federal regulations have been in place, a salad with chicken—not nuggets and french fries—is normal,” says Marlene Schwartz, director of the University of Connecticut’s Rudd Center for Food Policy and Obesity. “It’s exactly what lunch should look like.”

Ms. Miller Rubin is a writer in Chicago. Email her at reports@wsj.com.

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