On the headmaster's lawn he once mowed as punishment for violating the dress code, breach of decorum and other transgressions, Robert Caro is presenting a writing award his old high school has named for him.

"We had a very strict demerit system," the Pulitzer Prize-winning historian and former editor of the student newspaper says with a laugh about Horace Mann School, from which he graduated in 1953. "Week after week, I had the most demerits."

Five years ago, history department chair Barry Bienstock and school headmaster Thomas M. Kelly helped establish the Robert Caro Prize for Literary Excellence in the Writing of History. The reward is an intimate and informal luncheon held on the headmaster's property, across the street from the school, with Caro himself the featured guest. This year's winner is 17-year-old Yuanjun Zeng, known to all as Sarah, a junior who turned in a 23-page paper titled "The Mysteries Behind the Cuban Missile Crisis."

Sarah, who emigrated with her family from China when she was 12, said before Wednesday's ceremony that she knew little about United States history when she arrived. Caro's books on municipal builder Robert Moses and Lyndon Johnson, frequently taught at the elite private school, were part of her education.

"I thought it was really amazing how dedicated he was to his work," she said of Caro, whom she met for the first time Wednesday. "For example I heard he moved to Texas to do research and do work (on his Johnson books)."

Caro has a deep attachment to Horace Mann, in part because it was his dying mother's wish that he study here. And he takes special pleasure in the Caro Prize, designed to encourage the kind of deep research and vivid writing that he was praised for in such epic works as "The Power Broker" and "Master of the Senate." A school committee chose the winner, then sent the entry to Caro, who read through it and offered comments during his speech, when he praised Sarah for a "wonderful feat of writing" that made a complicated subject accessible.

Caro's 20-minute talk was like a Caro book in miniature — wide-ranging, digressive and informative. His subjects ranged from "The Iliad" to a Long Island family farm ruined by one of Moses' parkways. He spoke of the need for writing to matter as much in history as it does in fiction. He read a brief passage from "The Power Broker," his Moses book, and explained how rhythm could transform a list of road projects into a work of poetry.

"You have to write it so they read it," he said.

Caro, a longtime Manhattan resident, was lax in his day about such school requirements as having a pocket handkerchief. But the school now honors him. His books are included in an alumni display case on campus and a picture of the historian hangs in Bienstock's classroom, along with vintage photos of Johnson. The 80-year-old Caro remains in close touch with some of his former classmates and curious in general about the Bronx-based school.

After the ceremony ended, and a car waited to bring him and his wife, Ina, back to Manhattan, he made sure not to leave without upholding one last personal commitment.

He picked up a copy of the school paper.