NEW DELHI — Malls are open. Restaurants are packed. Markets are buzzing. As coronavirus cases plummet to their lowest levels in months, India’s lockdown feels increasingly like a thing of the past.

But 18 months after the country’s primary school students were sent home in March 2020, tens of millions remain out of school.

In the United States, students began returning to school in August, even as the delta variant surge in parts of the country delayed openings. In India, schools for older children have gradually reopened in recent months, but primary schools in more than half a dozen states have not. In major cities such as Mumbai and Delhi, at one point the country’s coronavirus hot spots, they remain closed.

Education professionals warn that the break in education threatens decades of progress in raising literacy rates.

The 2011 census, India’s most recent, recorded the national literacy rate at more than 73 percent, a jump of more than 20 percentage points from two decades before. The rate was higher among younger adults, a signal of the success of government efforts to provide universal free education. A yawning gap between male and female literacy, too, had narrowed.

But recent studies paint a grim picture of the impact of the extended school closures. Students from rural areas, where a majority of the country’s population resides, and those from marginalized communities, faced multiple barriers to continuing their education even before the pandemic.

A survey in August spanning 15 states found that 37 percent of children in grades one through eight in rural areas were not studying at all, and nearly 50 percent could not read more than a few words. Results in urban areas were only marginally better. The survey of nearly 1,400 children focused on students from underprivileged backgrounds who studied in public schools.

Reetika Khera, a professor of economics at the Indian Institute of Technology in Delhi who helped oversee the survey, worries that the scale of the problem isn’t widely understood. “These numbers suggest that an entire generation faces the real risk of remaining illiterate,” she said. “Whether the risk materializes depends on whether we wake up and take remedial measures.”

The survey detailed many of the challenges confronting children. A lack of smartphones and problems with connectivity plagued many households. In several instances, schools or teachers were remiss in sharing study material online. Young children whose parents hadn’t completed their own educations faced particular difficulty in understanding online classes. Other studies produced similar worrying findings.