With a mix of optimism and trepidation, teachers and students began returning to classrooms in large numbers Monday, as the nation opens a third straight school year upended by the pandemic.

Educators are anxious to begin academic and emotional recovery for students knocked about by more than a year of online and hybrid learning. But as a resurgent virus inflicts fresh damage, many were forced to confront a new round of pandemic politics, with debates over vaccines and masks consuming districts and communities.

The big fear lingering: a repeat of last year, when many students were forced to learn from home all or part of the time and students were regularly shuttled into quarantine after exposures to the virus.

The current surge, driven by the delta variant, has elevated case counts and hospitalizations across the country. There are about twice as many cases today as there were when schools began a year ago, when the country was coming off a case surge.

“Things are increasingly chaotic,” said Sasha Pudelski, advocacy director for AASA, the School Superintendents Association. “A month ago there was a lot more calm and assurity.”

She said district leaders are reevaluating policies on buses, masks, quarantining and more. “The concerns about delta have escalated quite quickly in some parts of the country.”

Already, some districts have pushed back the start date for school as local caseloads surge, and some were recording double-digit case counts of infected students and staff.

In Dallas, the superintendent on Monday defied an order from Texas Gov. Greg Abbott (R) and announced that students, staff and visitors will be required to wear masks in school buildings and on buses as coronavirus cases spike across the region and state. Abbott had issued an executive order barring government entities, including public schools, from mandating masks or vaccines.

Politicians and experts are weighing options to impose mask protocols and local vaccine mandates to effectively curb spread of coronavirus. (Blair Guild/The Washington Post)

“With numbers getting significantly worse, this decision is urgent and an important one when it comes to protecting our students, teachers, staff and their families,” said Michael Hinojosa, superintendent of the Dallas Independent School District.

The mandate takes effect Tuesday.

In Las Vegas, students at Matt Kelly Elementary School were welcomed back with cookies, lollipops, backpacks full of supplies and a DJ blaring dance music and Michael Jackson tunes. Teachers cheered as students and parents walked a red carpet from the curb into the school lunchroom, where breakfast was waiting.

First in line was kindergartner Levi Williams, who danced his way along the red carpet as his mother, Tina Williams, followed behind. “He’s my little prince,” she said, laughing.

But she is scared. “The classrooms aren’t well ventilated. And they’re small. This is a contagious disease. Just dealing with the common cold is hard, especially for young kids,” she said. She welcomes the state-mandated masks but knows how hard it will be for young children like Levi to keep his on.

“He’ll wear it for a couple hours but then he’ll get tired,” she said. “I say, ‘Don’t take it off. Don’t take it off.’ Once he removes it he doesn’t want to put it back on.”

This week, about one in four U.S. students begins school, according to tracking from Burbio, a data firm. By week’s end, about 36 percent of children will have begun, including those who started earlier.

Unlike a year ago, virtually all school districts have said they plan to offer full-time, in-person school, including the big cities that struggled throughout the past school year to reopen classrooms. Schools have been bolstered by tens of billions of dollars in federal funding to support safety measures and help students catch up. And while there was significant resistance by teachers to in-person classes last year, an estimated 90 percent of teachers have been vaccinated, and the two large unions are both signaling openness to vaccine mandates for school staffs.

“We are in far better shape today than we were last September,” said Randi Weingarten, president of the American Federation of Teachers. “The vaccine has been a game changer.”

In the affluent suburban community of Williamson County, Tenn., just south of Nashville, masks were optional and few students arriving for the first full day at Brentwood High School on Monday were wearing them. As the sun shone and humidity began to rise early Monday morning, 15-year-old sophomore Tyson Moody was all smiles, rolling toward school on a bright-blue moped.

“I just want to finish the school year and be able to play sports,” said Moody. He felt thankful that his football team was able to hold a scrimmage Friday evening. “It felt normal, and it really helped. It felt like a traditional Friday night in the fall.”

Moody is vaccinated but said he keeps a mask handy.

“I think my pastor said it best at church yesterday,” he said. “If someone feels more comfortable with you wearing a mask, just do it. It’s not being political. It’s just courteous.”

Fear — of illness, of a return to virtual learning — loomed for many.

In Prince George’s County, Md., Vedtra Gregg was longing for a virtual option for her seventh-grade son, something her district is not offering. Her fear of the virus persuaded her to get the coronavirus vaccine for her son and herself — despite her hesitancy. But she still worries about his ability to keep a mask in place for the entire day.

“Kids are going to be kids. Kids can be somewhat uncontrollable,” she said. “With this delta variant, you just can’t take chances.”

In Dallas, Nora Milan was also worried. She watched her 11-year-old son, August, fidget with his black face mask as he waited for the doors to open on the first day of fifth grade Monday outside Jack Lowe Sr. Elementary School.

“Pull it back up over your nose,” his mother commanded.

Lowe Elementary is one of 46 Dallas schools to start early in an effort to create additional instructional days for children who need it most.

She is excited for her son to get back to school. His short attention span made it hard for him to concentrate and learn remotely at home, she said.

“However, I am very worried now with this surge in covid cases,” she said. “I told him over and over that he has to keep his mask on and wash his hands, a lot. I don’t want him bringing home and spreading the virus.”

The mask battles were playing out all across the country. The Centers for Disease Control and Prevention recommends that all students and staff wear masks in school, saying masks are one of the most effective mitigation strategies available. But seven states have barred schools from doing so, while other states have statewide mandates in place.

In Florida, Gov. Ron DeSantis (R) threatened to withhold funding to districts that implement mask mandates, saying masking is a matter of personal choice. The state school board made private school vouchers available to students who live in districts with such mandates.

A handful of Florida districts are going ahead with requirements anyway. On Monday, Leon County Schools Superintendent Rocky Hanna announced masks will be required in K-8 buildings when schools open Wednesday. Only students with a doctor’s note will be exempted.

Hanna had opposed mandates but changed his mind as covid-19 rates skyrocketed in the county and four children and a few teachers were hospitalized recently.

“We know that social distancing is going to be tougher because we have a lot more kids physically returning to brick and mortar, which is where I want them,” he said Monday. “But I want them safe. And we need them safe. And I think again, in the abundance of caution, it would be prudent for us to just start this way.”

In Arkansas, Gov. Asa Hutchinson (R) signed a ban on mask mandates only to regret it. He asked the legislature to allow schools to impose requirements, but lawmakers refused. In the meantime, the entire measure was put on temporary hold Friday by a county judge.

“We are in many ways back where we were last fall or late summer, where districts had made plans to open up in person and things were changing quickly,” said Robin Lake, director of the Center on Reinventing Public Education at the University of Washington at Bothell. “Everyone was madly making adjustments and there was a lack of good guidance coming from states about what districts should be doing.”

All that, she said, is further complicated by mask and vaccine politics. “It’s making me very nervous. And it’s disappointing to me that we’re not better prepared at this point than we were last year.”

In Lamar County, Miss., two high schools moved to virtual classes after just a week of school in response to covid outbreaks. Both schools will be remote-only until next week, the district said, and all extracurricular activities were canceled.

Schools in Carter County, Ky., were supposed to begin last week, but the start date was pushed back to Aug. 16 “due to the increased number in Covid cases among school-aged children,” the district said.

Just a couple of days into the school year, on Friday the Gwinnett County, Ga., Public Schools, outside Atlanta, tallied 253 confirmed coronavirus cases. Together, eight metro Atlanta school districts reported 898 coronavirus cases in the first days of the new year, the Atlanta Journal-Constitution reported.

Nonetheless, teachers are opening their classrooms hopeful after what was for many the toughest school year of their careers.

“I’ve been teaching for over 20 years and last year was harder than my very first year of teaching,” said Las Vegas teacher Melissa McNally, who works with gifted and talented students at Ferron Elementary School. “I had to completely change everything that I did.”

McNally said she’s spoken to the parents of a number of her students and their reaction to their children returning to the classroom “runs the full spectrum.”

“You have parents who are concerned and cautious, and you have parents who are excited and will take any precaution that it takes for them to get back in the building,” McNally said.

McNally said she and her colleagues are aiming to make the coming year as normal as possible. “I’m happy to see their faces again in real life. I’m happy that they’re excited to come back to this building and I know they’re excited to see their friends and teachers again. I want this to be a magical year for them.”

Valerie Strauss contributed to this report. Nevins reported from Dallas, Slattery from Las Vegas, and Gee from Nashville.