BETHLEHEM, Pa. — Three hours into a recent Monday morning, blood had already been spilled in a hallway at Liberty High School. With his walkie-talkie in hand, the principal, Harrison Bailey III, called on the custodial staff to clean up the remnants of a brawl while hurrying to the cafeteria in hopes of staving off another.

This is how Dr. Bailey has spent many of his hours since the school welcomed back its 2,800 students for in-person learning in August: dashing around the 400,000-square-foot building, outrunning bells and crowds of students, and hoping that his towering presence will serve as an inspiration to pull up masks and a deterrent to other, less obvious burdens that his students have had to contend with since returning.

Like schools across the country, Liberty has seen the damaging effects of a two-year pandemic that abruptly ejected millions of students from classrooms and isolated them from their peers as they weathered a historic convergence of academic, health and societal crises. Teenagers arguably bore the social and emotional brunt of school disruptions.

Nationally, the high school-age group has reported some of the most alarming mental health declines, evidenced by depression and suicide attempts. Adolescents have failed classes critical to their futures at higher rates than previous years, affecting graduations and college prospects. And as elected leaders and public health officials scrambled to bring students back to school last winter and spring, the focus on having the youngest and most vulnerable students return to in-person instruction left many high school students to languish, with large numbers missing most or all of the 2020-21 academic year.

And now schools like Liberty must brace for an omicron-fueled wave of new infections, adding still more uncertainty.

On a recent day, as Dr. Bailey stood in one of Liberty’s busiest hallways — nicknamed the Hall of Fame for its frequent disruptive episodes — he described how the resignation and indignation that students brought back to school this year was palpable.

“It’s like there’s a bomb somewhere,” he said. “And you’re just hoping no one lights a match.”

Throughout the fall, the effects of the coronavirus pandemic have rippled through Liberty, a diverse regional high school in the Lehigh Valley in Pennsylvania, in the city of 75,000 where the famous Bethlehem Steel was founded.

The school’s wellness center has been overwhelmed with students struggling with anxiety and depression since the first day back. By the end of September, fights were frequent, and “blunt and flagrant disrespect” was rampant, Dr. Bailey said. In October, homecoming pep rallies were canceled for freshmen and sophomores, partly to follow Covid-19 restrictions and minimize crowds. By November, the principal was averaging at least one “informal hearing” per day for students who had been suspended.

By December, referrals for the school’s Student Assistance Program — in which teams of counselors and administrators coordinate resources for troubled students — had reached 300, compared with a total of 500 for the entire 2019-20 school year. At a recent meeting, where administrators sifted through their caseloads of “sapped” students, they described them in blunt terms: “feral,” “a mess” and “work in progress.”

“I think kids are just feeling like — after witnessing Trump, political unrest, what happened in the streets with Black Lives Matter, now the pandemic — the world’s out of control,” Dr. Bailey said. “So they’re like, ‘The world’s out of control, why should I be in control?’”

Liberty’s staff is not faring much better. Only a handful of teachers have taken a formal leave of absence, but they are not whom Dr. Bailey worries about most. He is concerned with the ones “right on the edge.”

He has noticed that some teachers with strong classroom management skills are sending their first referrals to his office. Some of the most engaged staff members — those who have volunteered to lead clubs — have had to pull back to focus on new challenges in their classrooms or their own lives. And for some, the “acting out” among some students is far less concerning than the sheer apathy they have encountered.

“For the teachers, like all of us, they’re here for the kids, not the money,” Dr. Bailey said. “So to have a higher number of kids you can’t reach, it’s intense.”

While conflicts over coronavirus response strategies like masks and quarantines have dominated reopening debates, school leaders say it is the day-to-day tasks of running a school building that have brought the most turmoil.

And while much attention has been paid to besieged superintendents and burned-out teachers, the responsibility to restore a sense of normalcy has largely fallen on principals.

At the beginning of the school year, Dr. Bailey, who has led Liberty for a decade and is the state’s principal of the year, told his staff that their mission was to survive. He warned that the year “would be the most difficult time to be in education since we sent kids to Vietnam.”

At Liberty, vestiges of remote learning linger. Many students wear pajamas, the dress code of bedrooms turned to classrooms and a reflection of disrupted sleep schedules. Students move through the hallways sluggishly, looking at their phones or straight ahead, as if still staring at computer screens.

Last year, 66 percent of students were hybrid learning, and more than 33 percent went completely virtual. Students and educators use terms like “re-entry,” “recivilizing” and “reintegrating” to describe the transition back to a more normal routine. Covid restrictions still prevent full engagement. Masks have encouraged anonymity and discouraged dialogue.

“People don’t know how to communicate anymore,” said Jazlyn Korpics, 18, a senior at Liberty. “Everybody’s a robot now — their minds are warped.”

Josiah Correa, 18, said that while he was a senior at Liberty, “every day it feels like I’m starting a new school.”

For Nikolas Tsamoutalidis, an assistant principal, the most vivid image of the post-pandemic student body was at lunch this year, when he saw ninth graders — whose last full year in school was seventh grade — preparing to play “Duck, Duck, Goose.” “It’s like fifth or sixth graders,” he said, “but in big bodies.”

The cheeriest part of the school is the wellness center, with social workers, therapists, bean bag chairs and soothing paint colors. Dr. Bailey used grant dollars to build it a year and a half ago as part of his plan to make Liberty a “trauma-informed school.” Even before the pandemic, the district was looking to use the center as a model for addressing the mental health crisis brewing in Bethlehem and beyond.

Nancy Ettwein, who ran the wellness center until November, said that the need for services at the beginning of the school year was “off the Richter scale.”

“The No. 1 thing is anxiety,” she said in September. “Anxiety about being in the classroom, being in front of people, speaking to people, anyone looking at them.”

Robin Sorensen, the wellness center’s clinical supervisor, said the school would be “lost” had Dr. Bailey not created the space. The four therapists’ caseloads are nearly full. “I’ve never seen more referrals for mental health that just say, ‘Sitting and crying in the bathroom,’” Ms. Sorensen said.

Kaisyn Carswell, 16, filed in on a recent day after he came across someone being jumped in the boys’ bathroom. The center, which he visits several times a week for therapy and “breaks,” has helped him weather life during the pandemic, which he described as “when you feel emptiness, but the emptiness is really heavy.”

Dr. Bailey’s 75th “informal hearing” of the year illustrated perhaps his biggest challenge with the pandemic-era student body.

He had been preparing to offer a ninth-grade student “cyberschooling,” as it is called here, after the student had been suspended for three days.

The student had been late 29 times, and had 12 absences and 63 class cuts. As Dr. Bailey read out the freshman’s single-digit class averages, his voice changed when he got to 53 percent in U.S. history.

“Wow. You’re smart! How do you never go to class and get a 53?” he said.

The student responded that school was not challenging, and that in middle school it had been easy to do just well enough to play sports. Now the student had no interest in joining Liberty’s sports teams, and didn’t see the point in attending class; it was more important to get a job and make money for a future family.

“What did you do last year?” Dr. Bailey asked.

“Just played video games all day,” the student responded.

Instead of referring the student to remote school, Dr. Bailey advised starting fresh the next morning. He ended the hearing with a hopeful handshake but walked back to his office defeated.

“We’re running out of Band-Aids,” he said. “The schools are bleeding out, and it feels like no one is listening.”

Across the country, principals are echoing Dr. Bailey’s distress.

Survey results released this month by the National Association of Secondary School Principals raised alarms that the pipeline of principals might be another casualty of the pandemic, as their roles grow more amorphous and untenable.

In responses to the survey, which included a nationally representative sample of leaders, only 35 percent said they “strongly agree” with being generally satisfied in their jobs, down from 63 percent in 2019. Ranking among the highest on their list of challenges during the pandemic was providing mental health support to students and providing guidance and mental health support to staff. Sixty-eight percent were worried about teacher shortages and teacher burnout.

Only 23 percent “strongly agree” that the size of their administrative team is adequate to support staff and students, and only 21 percent “strongly agree” that there are adequate student services personnel like nurses and counselors.

While funding and political will were seen as the answers to reopening schools, they have done little to solve real-time issues like labor shortages and a drought of community services, which have been exacerbated by the pandemic.

“The thing that is really frustrating our members is that they can see that there are resources out there in the world,” said Ronn Nozoe, the chief executive of the National Association of Secondary School Principals. “And it burns them to the core that they don’t have the systems and structures and processes and power to marshal that all together to serve their students.”

“They know every single day, students, parents and educators are struggling, and they’re trying to do everything they can,” he added. “And they can’t be everything to everybody.”

Staff members fear that Dr. Bailey, 49, may kill himself trying.

“I worry to keep us from falling apart, he keeps it all in,” said Fred Harris, the school’s athletic director, who has known Dr. Bailey for 12 years.

At an administrative hearing in December, Dr. Bailey tried to calmly steer his team to winter break. The agenda for the meeting was 10 items long; as they moved down each one, administrators grew more exasperated.

There was a new directive from the district that required testing athletes, but Dr. Bailey still needed the school’s athletic director to cover three lunch periods.

“I can’t do it,” Mr. Harris said. “I’m only one guy.”

There were 185 students failing 348 classes and tension over what to communicate to teachers with the highest numbers of failing students.

“Tell them you have the freedom to be creative,” Dr. Bailey told his team. Do what you have to do to get students to pass.

“I’m concerned we’re going to give the impression that all kids deserve to pass,” one of his colleagues shot back.

The school district had proposed a plan, based on an increase of “volatile incidents,” to transfer students to online schooling as an alternative to expulsion.

“It’s antiacademic, and honestly this is about trying to salvage an environment for adults,” an assistant principal said of the plan.

On top of that, the school was bracing for its first student mask exemption, just as coronavirus cases were surging after Thanksgiving.

“Her mom’s yelled at me. Her dad’s yelled at me. She’s not going to wear the mask,” an assistant principal said. “What do we do if other students and teachers don’t want to be around her?”

“Let’s deal with that when it happens,” Dr. Bailey replied. “It’ll be stormy, but we’ll weather it. We always do.”

Sarah Mervosh and Erin Schaff contributed reporting.