The study, reported in the journal Clinical Infectious Diseases, looked at what happened in two Texas communities during the H1N1 "swine" flu epidemic of 2009. In one community, schools were closed as a precaution; in the other, they weren't.
It turned out that in the district where schools shut down, there were fewer ER visits for the flu.
What's more, among kids age 6 and up, there was no increase in flu-related ER trips, while that rate doubled in the community where schools stayed open.
"The effect was most dramatic among school-age children," said Dr. Martin S. Cetron, of the Centers for Disease Control and Prevention (CDC).
There have been skeptics who've doubted that school closures could have much impact during a major flu outbreak, according to Cetron.
"They've said, well, people will just congregate in malls or other public places," explained Cetron, who directs the CDC's division of global migration and quarantine, and worked on the study.
But schools are different from malls, Cetron pointed out, with kids being in close contact with each other all day long.
He said he thinks this study, along with others, "settles" the question of whether school closures are effective. "Should this be an arrow in our quiver? I think the answer is 'yes,'" Cetron said.