Though no two schools are the same, there are many reasons school environments can prevent excessive weight gain. Most schools offer students some level of exercise through physical education, team sports, and daily lunch and recess periods. Many children get about half their daily calories at school, which are required to provide meals that meet nutrition standards for components like sodium, whole grains, lean protein and fruits and vegetables. While junk foods can still be found in school cafeterias, many schools have worked to eliminate sugary beverages, candy and chips from their lunch lines and vending machines.

During the summer vacation, it’s the reverse. Studies find that children spend more time sitting in front of screens watching television and playing video games. They tend to consume more snacks and sugary beverages and eat fewer fruits and vegetables. That may especially be the case for children from low-income households that depend on schools to provide healthy meals. About 30 million children across the country receive free or subsidized school meals.

Dr. Rundle and his colleagues suggested in their new paper that schools, parents and policymakers could mitigate the long-term impact of the pandemic on children’s health by promoting exercise and healthy eating — where possible — during the lockdowns. Some schools, for example, have developed home lesson plans for exercise to go along with their lesson plans for math and English. Schools that are able to stream classes online might consider having their P.E. teachers stream exercise classes too, Dr. Rundle said.

Food is the trickier part of the equation. For some families right now, venturing outside to find healthy options at depleted grocery stores may not be possible. And with many parents stressed about their jobs, finances and other challenges, the prospect of fighting with their kids about food can be a hard sell — especially if they are worried about being able to put any food on their tables at all.

But with children facing limited options for physical activity, now more than ever is the time to try to limit unnecessary calories from sugar-sweetened beverages and excessive snacking, said Dr. Rundle. One good resource for stressed families is the American Heart Association, which provides quick, heart-healthy recipes on its website — like tuna stir fry, chicken salad, hummus and vegetarian three-bean chili — that can be made with canned foods and other inexpensive pantry items.

For families that rely on school meals to feed their children, many school districts across the country have been providing grab-and-go meals. Some are offering five meals at a time and allowing parents to pick up food without their children being present. In some districts, schools are packing up meals and having bus drivers deliver them to families along their normal routes.

Parents can find out where to get free meals through their school districts. They can also find meal service sites in their neighborhood using the federal government’s Summer Food Service Program website. For many children, these services could mean the difference between having a nutritious lunch and breakfast or not having any food at all, said Eliza Kinsey, a co-author of the new paper and a research scientist in the department of epidemiology at the Mailman School of Public Health.