Students in some places have hiked for hours and braved the cold to listen to online classes on mountaintops, the only places they can get a decent cell signal, according to Chinese news reports. One high schooler in Sichuan Province was found doing homework under a rocky outcropping. Two little girls in Hubei Province set up a makeshift classroom on a wooded hillside.

For children of the millions of migrant laborers who work far from home to keep China’s cities cleaned and fed, another problem is a lack of supervision. These “left-behind children,” as they are called in China, are raised mostly by their grandparents, who are often illiterate and cannot help with homework even when it is not delivered via smartphone app.

Wang Dexue, an elementary school principal in hilly Yunnan Province, said that in some classes, half the students cannot participate in online lessons because their families lack the necessary hardware.

For households that can connect, parents are not always invested in helping their children with remote learning, Mr. Wang said. His teachers are still figuring out how to teach with video apps. “Teaching progresses much more slowly sometimes,” Mr. Wang said.

The virus has come at a delicate moment for China’s efforts to help its least fortunate. This is the year the Communist Party has vowed to eradicate extreme poverty. The country’s top leader, Xi Jinping, has held fast to that goal despite the public health emergency. But raising people’s incomes above the level of deprivation was never going to be as tough as providing them with better educational opportunities.