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As school districts around the United States debate when and how to reopen schools, a large new study from South Korea offers a note of caution. It found that children between the ages of 10 and 19 can spread virus at least as well as adults do, suggesting that middle and high schools in particular may seed new clusters of infection.

Children younger than 10 transmit to others much less often, the study found, although the risk is not zero. That is consistent with what many other studies have reported.

Several experts said that the study was carefully done and that the results suggested schools should have concrete plans in place for dealing with outbreaks before reopening.

“I fear that there has been this sense that kids just won’t get infected or don’t get infected in the same way as adults and that, therefore, they’re almost like a bubbled population,” said Michael Osterholm, an infectious diseases expert at the University of Minnesota.

“There will be transmission,” Dr. Osterholm said. “What we have to do is accept that now and include that in our plans.”

The South Korean researchers identified 5,706 people who were the first to report Covid-19 symptoms in their households from Jan. 20 to March 27, when schools were closed. They then traced the 59,073 contacts of these “index cases.” They tested all of the household contacts of each patient, regardless of symptoms, but only tested symptomatic contacts outside the household.

On Wednesday, a panel from the prestigious National Academies of Science, Engineering and Medicine recommended reopening schools wherever possible for disabled children and for those in elementary schools, because those groups have the most trouble learning online. Caitlin Rivers, an epidemiologist on that panel, said the new study did not alter that recommendation.