“I want to serve the students, but it’s hard to say you’re going to sacrifice all of the teachers, paraprofessionals, cafeteria workers and bus drivers,” said Hannah Wysong, a teacher at the Esperanza Community School in Tempe, Ariz., where virus cases are increasing.

School systems struggling to meet the financial and logistical challenges of reopening safely will need to carefully weigh teachers’ concerns. A wave of leave requests, early retirements or resignations driven by health fears could imperil efforts to reach students learning both in physical classrooms and online.

On social media, teachers across the country promoted the hashtag #14daysnonewcases, with some pledging to refuse to enter classrooms until the coronavirus transmission rate in their counties falls, essentially, to zero.

Now, educators are using some of the same organizing tactics they employed in walkouts over issues of pay and funding in recent years to demand that schools remain closed, at least in the short term. It’s a stance that could potentially be divisive, with some district surveys suggesting that more than half of parents would like their children to return to classrooms.

Big districts like San Diego and smaller ones, like Marietta, Ga., are forging ahead with plans to open schools five days per week. Many other systems, like those in New York City and Seattle, hope to offer several days per week of in-person school.

Adding to the confusion, optional guidelines released by the Centers for Disease Control and Prevention in May set out ambitious safety precautions for schools. But the president, and many local school system leaders, have suggested they do not need to be strictly followed, alarming teachers.

Many doctors, education experts, parents and policymakers have argued that the social and academic costs of school closures on children need to be weighed alongside the risks of the virus itself.

The heated national debate about how and whether to bring students back to classrooms plays upon all the anxieties of the teaching profession. The comparison between teachers and other essential workers currently laboring outside their homes rankles some educators. They note that they are paid much less than doctors — the average salary nationwide for teachers is about $60,000 per year — but are more highly educated than delivery people, restaurant workers or most staffers in child care centers, many of whom are already back at work.

Now, as teachers listen to a national conversation about reopening schools that many believe elevates the needs of the economy and working parents above the concerns of the classroom work force, many are fearful and angry. They point out that so far Congress has dedicated less than 1 percent of federal pandemic stimulus funds to public schools stretching to meet the costs of reopening safely.

The message to teachers, said Christina Setzer, a preschool educator in Sacramento, is, “Yes, you guys are really important and essential and kids and parents need you. But sorry, we don’t have the money.”

Earlier in the shutdown, Mr. Trump acknowledged the health risks to teachers over the age of 60 and those with underlying conditions, saying at a White House event in May that “they should not be teaching school for a while, and everybody would understand that fully.”

But this week, as the administration launched a full-throated campaign to pressure schools to reopen in the fall — a crucial step for jump-starting the economy — it all but ignored the potential risks teachers face. More than one-quarter of public schoolteachers are over the age of 50.

Teachers say many of their questions about how schools will operate safely remain unanswered. They point out that some classrooms have windows that do not reliably open to promote air circulation, while school buildings can have aging heating and cooling systems that lack the filtration features that reduce virus transmission.

Although many districts are spending millions this summer procuring masks, sanitizers and additional custodial staff, many teachers say they have little faith that limited resources will stretch to fill the need.

They also worry about access to tests and contact tracing to confirm Covid-19 diagnoses and clarify who in a school might need to isolate at home in the event of a symptomatic student or staff member.

The C.D.C. has advised against regular testing in K-12 schools, but on Wednesday, Dr. Deborah L. Birx, the White House coronavirus response coordinator, said the Trump administration was exploring whether testing being developed for other vulnerable environments, like nursing homes, could be used in schools.

Indeed, educators have had to process a head-spinning set of conflicting health and safety guidelines from Washington, states and medical experts.

The C.D.C. has recommended that when schools reopen, students remain six feet apart “when feasible,” while the American Academy of Pediatrics released guidelines suggesting that three feet could be enough space if students wore masks.

But after major pushback from educator groups, who felt there was too little attention on the health risks for adults who work in schools, the Academy joined with the two national teachers’ unions on Friday to release a statement saying, “Schools in areas with high levels of Covid-19 community spread should not be compelled to reopen against the judgment of local experts.”

In Arizona, Ms. Wysong, 30, said she was willing to return to her Tempe classroom; she is not in a high-risk category for complications from Covid-19 and her school caps classes at 15 students. But given the long-term teacher and substitute shortage in Arizona, which has some of the lowest educator salaries in the nation, she said she believed the overall system could not reopen safely with small enough class sizes.

Health and education experts who support reopening schools have sometimes questioned the need for strict physical distancing, pointing in recent weeks to emerging research suggesting that children may be not only less likely to contract Covid-19, but also less likely to transmit it to adults.

In interviews, many teachers said they were unaware of or skeptical of such studies, arguing that much about the virus remains unknown, and that even if teachers do not catch coronavirus in large numbers from children, it could be spread among adults working in a school building, or during commutes to and from schools via public transit.

The education systems in Germany and Denmark have successfully reopened, but generally only after local virus transmission rates were brought under control.

American schools currently have a variety of plans for welcoming students back to campuses, ranging from regular, five-day schedules with children using desk partitions to stay distanced, to hybrid approaches that seek to keep students physically distanced by having them attend school in-person only a few days per week, and spend the rest of their time learning online from home.

In New York City, Mayor Bill de Blasio announced last week that the nation’s largest school system would reopen only part-time for students this fall, but teachers would most likely be back in classrooms five days a week.

The teachers’ union president, Michael Mulgrew, has said he does not believe schools can reopen at all if the city does not receive additional federal funding this summer.

With many teachers reluctant to return to work, according to polls, staffing will be a major challenge for districts across the country. New York estimates that about 1 in 5 of its teachers will receive a medical exemption to teach remotely this fall.

Matthew Landau, a history teacher at Democracy Prep Charter High School in Harlem, hopes he will be one of them. He survived stage four cancer several years ago and said he does not feel comfortable going back to his classroom.

“I feel there’s no way to keep immunocompromised teachers safe,” he said.

Kevin Kearns, a high school English teacher at the High School of Fashion Industries in downtown Manhattan, has spent the last few weeks wrestling with his own dilemma.

Mr. Kearns and his wife became parents in March, and need child care for their infant son. Their only option is to have Mr. Kearns’ mother-in-law, who is in her 70s, stay with them. Mr. Kearns is terrified of bringing the virus home.

“I don’t want to go back, I don’t think it’s safe to go back, but I don’t know that I necessarily have a choice,” he said.

Still, Mr. Kearns said he feels a duty to the mostly low-income, Black and Latino students he teaches.

“It puts me in a very difficult moral conundrum,” he said, “to choose between supporting my community, students, colleagues and my own family’s safety.”

Erica L. Green contributed reporting.