NEW YORK — Madeleine Brennan maintains Dyker Heights Intermediate School 201 in Brooklyn as something of a time capsule.

Female secretaries, guidance counselors and assistant principals are asked to wear dresses or skirts; teachers may wear slacks, but not dungarees; men all wear ties. The marble staircase shines; the hallways are painted a classic pale blue. Each year before Christmas, there is Rhinestone Week, in which Brennan encourages staff members to rummage through their grandmothers’ things for old costume jewelry to wear.

But the prize artifact of the past is Brennan herself, who has been principal of the school for 48 years, longer than most of her teachers have been alive — longer, experts believe, than any other principal in the country. When she first arrived to work at this imposing brick building in March 1963, John F. Kennedy was president, ZIP codes were not yet in use, and the nearby Verrazano-Narrows Bridge was still under construction.

She has outlasted more than a dozen schools chancellors, who made what she described as “little changes here and there,” and watched a student body dominated by the children of Italian immigrants transform into one that is 45 percent Asian-American and 18 percent Hispanic.

But as the city embarks on an overhaul of its middle schools, Brennan believes that what works remains the same. Consistent rules and consequences. A dedicated, hard-working staff. A calendar stuffed with activities like a Shakespeare fair and an annual musical. Sincere care for your charges.

“Teenagers fascinate me,” Brennan said in an interview in her pin-straight office. “They are peculiar ducks, neither fish nor fowl. And you have to love them to really work with them. If you don’t love them, you are up a tree.”

The city principals’ union plans to honor Brennan today as the only leader who has been around nearly as long as the 50-year-old union itself. “It’s almost dizzying when I think about how many students and educators Madeleine has inspired over the course of her long career in New York City public education,” said Ernest Logan, the union’s president.

She earns $148,000 a year, but with her pension and 401(k), she would make more by retiring. “I don’t care,” she said. “If you like what you are doing, you can do it for a lifetime.”

Brennan — who stands under five feet tall, with rouged cheeks, vaguely pink set hair, and chunky clip-on earrings — keeps her age a closely guarded secret. Suffice it to say that her first day of work as a New York City teacher, according to school records, was in 1946.