It is a sweltering August day, and Joanne Walsh is apologizing for not keeping her office cool.

She is living the principles of Pope Francis’ message of caring for our common earthly home, she said, so she’s keeping the air conditioning to a minimum. Also, if she runs two air conditioners on the same circuit at the same time, she will blow a fuse.

The wiring problem is a nagging worry for Ms. Walsh, the 61-year-old principal of Our Lady Queen of Angels in East Harlem. The Roman Catholic school, whose students range from prekindergarten through eighth grade, will be a stop on the pope’s two-day sweep through New York City next month.

“Nobody plugs in anything,” she said she warned organizers of the papal visit. “Not even a cellphone.”

That Ms. Walsh is thinking about details like electrical sockets—amid a mounting list of back-to-school work and the pressure of hosting a world leader—is no surprise to colleagues past and present.

“I’m sure she’s a big part of the reason why they chose that school. You won’t see one kid out of line, one hair out of place,” said the Rev. Mark Cregan, former pastor of Sacred Heart in the Bronx, the parish home of the school where Ms. Walsh spent the first 25 years of her career.

Colleagues describe her as a workaholic, rarely taking vacations and unlikely to log less than a 12-hour day. They have seen her attend the funerals of students’ family members, lead school plays, mop floors and fix toilets.

“It can’t be a job. If this was a job, it would be impossible,” Ms. Walsh said. “It’s a life, and it’s a choice I made.”

As an educator, she fought for her students to have sophisticated science equipment and labs. Most important, colleagues say, she has run toward work in some of the city’s poorest areas.

“She is just remarkable in terms of seeking out those who needed us the most,” said SisterAnn Veronica Bivona, the principal of St. Margaret Mary School in the South Bronx and a colleague who has known Ms. Walsh for at least 20 years.

“That just comes right off her genes naturally. That’s who she is. She’s a woman that cares deeply about others and who is very deeply seated in her own Catholic faith,” said Sister Bivona.

Yet Ms. Walsh is no-nonsense when it comes to getting students to behave like little angels.

Allison Reyes, 8, who will be a third-grader at Our Lady Queen of Angels this fall, said she has never seen Ms. Walsh act silly.

“She doesn’t do anything funny,” said Allison. “She likes being serious.”

That wasn’t always the case. About 1½ years into her coursework at Lehman College, part of the City University of New York, Ms. Walsh suspended her studies, bought a used red Buick for $200 and drove cross-country solo. She camped outside and slept in her car.

The move was in line with being a “child of the ’70s,” she said. She was outspoken on social issues but retained her moral compass and regular baths, she joked.

The eldest of five children, born in the Bronx and raised in Dobbs Ferry, N.Y., Ms. Walsh had a Catholic upbringing and education. Her mother was a school nurse, and her father worked in advertising.

Her life plan was to be a social worker. But as her collegiate road trip took her to Oregon, she decided to become a teacher by the banks of the Columbia River Gorge. She finished college on time and took a volunteer teaching job on a reservation in North Dakota.

After that, in 1977, Ms. Walsh landed back in Westchester and took a job at a Korvette department store. Her plan was to begin teaching the following September.

She started even sooner, thanks to a chance encounter with an old teacher that led to the Sacred Heart job. She was 23 and “tripped through” her first teaching gig, she said, but the South Bronx was where she wanted to be, and she eventually became the school’s principal, overseeing hundreds of students.

During her tenure at Sacred Heart, Ms. Walsh spent two summers volunteering with schools in Liberia. Upon leaving Sacred Heart, she agreed to make a one-year commitment helping schools there and began saving money for a sabbatical. But safety concerns repeatedly delayed her plans.

All the while, she continued to get offers to consult or take another job. She set a deadline of Feb. 1, 2004, to make firm plans to get to Liberia.

The day before her deadline, Ms. Walsh got a call about the principal’s job at Our Lady Queen of Angels. She didn’t hesitate, knowing in her heart that it was the right move, she said.

When she arrived, the school was struggling and at risk of closure, she said. The parish attached to the school ultimately closed in 2007, sparking a public battle between some parishioners and the Archdiocese of New York.

Ms. Walsh was in charge of everything, from starting the boiler in the morning to paying bills and meeting with parents.

“If it’s too easy or too perfect, you’re not going to find Joanne there,” said Abigail Akano,who was hired by Ms. Walsh and now serves as principal of Sacred Heart. “Joanne loves to get into the nitty-gritty of things and work to make it better.”

By all accounts, Ms. Walsh has made her school better. In 2013, Our Lady Queen of Angels became one of six archdiocese schools to join with the Partnership for Inner-City Education, a nonprofit school-management organization focused on Catholic education. The six schools borrow innovations from the charter-school movement and are supported with money and operational assistance from Partnership Schools.

Ms. Walsh’s annual budget is about $2.7 million, with each student costing around $9,400 to educate. Nearly 70% of the 282 students receive some financial aid.

The added support from Partnership Schools has allowed Ms. Walsh to focus on her teachers and on raising academic standards, she said.

School officials hope the papal visit will draw attention and donations to archdiocese schools. But Ms. Walsh brushed off the suggestion that meeting Pope Francis will be the capstone moment to nearly four decades in education.

Doors opened to good jobs, she said, and nothing has been coincidence or luck.

“I believe in divinely coordinated encounters,” she said. “I have been blessed.”

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Write to Melanie Grayce West at melanie.west@wsj.com