More than 30,000 K-12 schools in the United States are being shuttered because of worries about spreading the coronavirus, affecting at least 20 million students, most of whom will be asked to shift to online learning. Though health experts disagree to what extent school closures will help, entire states and some of the nation’s largest cities, including Massachusetts and Boston, announced closings in recent days.

Educators experienced with remote learning warn that closures are a serious threat to children’s academic progress, safety and social lives. They say that running a classroom digitally is much harder than bringing an adult workplace online, and that it can disproportionately affect low-income students and those with special needs.

Here are some of the warnings and tips from various news sources that teachers from around the country, well-versed in remote learning, have for schools planning to move online.

Not every home has computers or high-speed internet.

The vast majority of households with children have broadband internet, but there are still big disparities by income, race and the education level of parents.

Low-income families are more likely to rely on smartphones for internet access, and children in those households may not be able to use more sophisticated learning software that requires a tablet or computer. It is not unusual, educators say, for siblings to try to complete their schoolwork on a single cellphone.

Nate Ridgway, a social studies teacher at Beech Grove High School near Indianapolis, regularly creates video lessons for his students. His school also provides Chromebooks that students can take home. But when his district had two “e-learning” days this year because of snow, he noticed that some disadvantaged students fell behind academically.

“We as educators have to be so, so careful about this expectation that we go completely online,” he said. “Anywhere from 10 to 25 percent of my students may not have internet access at home.”

Younger children require lots of adult supervision.

Younger students need help to learn online — lots of help. Parents may need to assist their child with turning on a device, logging into an app, reading instructions, clicking in the right place, typing answers and staying on task.

Even the tech-savviest adult will find this difficult while working from home at the same time — a more common scenario as the coronavirus spreads. Parents who continue to work outside the home when schools are closed will need to arrange child care, where technical help could be scarce. In Salinas, Calif., Ben Cogswell prides himself on his tech-savvy kindergarten classroom. He records videos of himself reading beloved children’s books and reviewing words via flashcards. Many of his students speak Spanish at home, and he hopes to strengthen their English skills outside school hours.

But access to these resources can be a challenge, he said. Even though the district provides all students with a Chromebook, not all parents — even those with home WiFi — know how to connect that device to the internet.

Even great teachers lack expertise in creating online lessons.

While there are lots of exceptional teachers, not all of them are ready to move their instruction online. Online lessons need to have more clearly written-out themes and directions for students, said Sarah Giddings, a teacher at WAVE, a high school in Ypsilanti, Mich., that blends online and in-person learning. “You can be a fantastic teacher, but writing curriculum is hard,” she said.

Education technology firms have aggressively promoted their products as school closures become more widespread. But educators who have pioneered online learning say some of the best tools — like Google Hangouts or Flipgrid, an interactive video platform — cost nothing.

“A teacher’s favorite price is free,” Mr. Ridgway said. He and other educators warn against using learning tools that make it difficult to log in, are inaccessible via mobile devices or require downloading special software. Remote learning, they say, should simply require signing into a website.

Students with special needs can be the hardest to teach virtually.

Christopher W. Bakk, a social studies teacher at Turning Point Academy in Racine, Wis., has taught special education students both in-person and remotely, via the Wisconsin eSchool Network. Some of those students have behavioral issues and thrive online because there are fewer social distractions, he said. But others find it difficult to have less direct access to teachers and peers. “The self-discipline is a struggle,” Mr. Bakk said.

In addition, many students in this generation are nervous about speaking over the phone, which Mr. Bakk said can be a crucial tool for teachers to check on students who are learning at home. He approaches those discussions with humor to help self-conscious teenagers relax.

“You have to ease their anxiety,” he said.

Schools provide more than academic skills.

Even when the devices, WiFi, software, lesson plans and adult supervision are all in place, a lot is lost when schools transition students to remote learning. Many children rely on schools for free or affordable meals, for counseling and for after-school activities while parents work.

When schools are closed, children lose a crucial social outlet. And families, especially those who work in the service sector and cannot easily adjust their schedules, can struggle to find appropriate child care. “If you think about it, the school is a city we provide to kids,” said Mr. Ridgway. When that city shuts down, he said, no online learning platform can replace all the structure and vibrancy that is lost.