When Principal Bill Lessa first walked through the doors of Cardinal Hayes High School as a social studies teacher in 1971, he never envisioned that he would one day become the school’s longest-serving educator.

Hundreds of students, faculty, alumni and friends packed into the Bronx school auditorium Thursday to celebrate Lessa’s retirement, a cap on an incredible 53-year career served entirely at the all-boys Catholic institution.

The Mass in his honor started and ended the same way — with a standing ovation that lasted several minutes as “Principal Lessa” chants echoed throughout the room.

“These kids are the only kids I ever wanted to be with, the only kids I wanted to work with,” Lessa, 75, told The Post after the ceremony.

“During my career, I had many opportunities to go other places, but somehow or another, I always found the wherewithal to stay here because you can see how much they appreciate what you do for them. And those kids don’t have a lot of support in a lot of areas. And I always felt that if I could be just a bit of a change agent in their lives, I would be doing something really noble.”

Lessa is the longest-serving educator at a single Catholic institution in the Bronx, according to the Archdiocese of New York — and one of the few who boast more than five decades at a school in all of New York State.

The educator started his storied career at Cardinal Hayes upon graduating from Iona College in 1971 — marrying his wife Mary Anne during the few short months in between.

He served as a social studies and religion teacher for three years, when he scooped up the additional responsibility of being a guidance counselor, spending decades helping his students navigate the college admissions system.

It was in 2008 that Lessa was asked to serve as the interim principal — a job that quickly became clearly his calling in life.

“I always wanted to be a teacher, but this has put me in a spot where it could be so impactful,” he explained.

Lessa’s love for his students has always been his major motivator. When asked what his biggest accomplishment over his 53-year career was, he emphasized that it was watching the alumni find their own success.

He pointed to alumni Willie Colon, a former New York Jets guard, and Dr. Jesus Jaile-Marti, a neonatal specialist in White Plains, as examples of his pride — and gushed that the pair were among many who have come back over the years to express Lessa’s influence over their phenomenal careers.

“I know that things might have been different if I hadn’t run into these guys. Our job is to be transformational. That’s what we’re supposed to do,” Lessa said.

“And it’s cyclical. You do it every year. You have another shot every year. It just goes on and on and on.”

Lessa has seen numerous phases of Cardinal Hayes throughout his five decades, and ushered the community through financial hardships that have forced dozens of other, neighboring Catholic schools to permanently shut their doors in recent years.

This year’s graduating class of roughly 150 young men is a far cry from the 450 who crossed the stage during Lessa’s first year.

While Lessa attributes the school’s survival to the determination of the students and their families, the school community pointed to him as the clear leader — especially considering he routinely pulled 12-hour days throughout his career,

“You’re a great example of faithful service, fighting strength and unwavering determination,” said Father Steven Gonzalez, the school’s chaplain.

Luckily for the Cardinal Hayes community, Lessa isn’t ready to say goodbye just yet.

Although he won’t serve as the school’s principal, he plans to work within the alumni network, putting his more than 50 years of alumni connections to good use.

Lessa hopes to start soon, and has no plans to take a vacation before starting his new gig.

“This has been my life for almost 54 years … and I’m not a ‘relaxing’ guy,” Lessa said.

“I find I always find something to do, especially if it’s good for this particular institution.”