D.C. School Superintendent Floretta D. McKenzie announced her resignation yesterday after six years as head of the city's public school system, saying she had decided to establish an educational consulting company in partnership with a major Washington law firm.

She will leave her $85,000-a-year post after the fall semester, which ends Jan. 29.

McKenzie, whose intention to resign was first reported a week ago, told her plans to the school board at a special private meeting yesterday morning.

In a session later with reporters, the 52-year-old superintendent declared: "I love the city, I love the {school} staff. I love what I'm doing . . . . But I have been here for six years, and I just think it's time to move on and give somebody else a chance." Then she added, "And there are other things I plan to do. I think it's important . . . to leave when it is all going well."

A cautious, methodical administrator, McKenzie projects a calm, sometimes folksy image in public, although in private, school officials said, she is often blunt and an unabashed workaholic.

The new consulting firm, called the McKenzie Group, is scheduled to open late this year, according to a statement McKenzie distributed. It will be affiliated with Hogan & Hartson, one of the city's largest law firms, which is investing funds in the venture and will sublet space to it in its downtown offices.

"I never learned to do this {superintendent's} job easy," McKenzie remarked. "It has always consumed me, and one wonders how one can keep up the same pace well into seven or more years."

Board of Education President R. David Hall, who sat beside McKenzie at the news conference, said he was "very sorry about her decision" and praised her as a "tremendous asset to our school system." He added that the schools "will not miss a beat" when fall classes begin Sept. 8, and said the board will establish a plan within a month for selecting a new superintendent.

Other school board members and city officials also praised McKenzie yesterday. D.C. Council Chairman David A. Clarke said in a statement, "it is our children who stand to lose the most" because of McKenzie's departure. He urged her to stay at least until the school year ends in June.

But at the meeting with the school board, McKenzie indicated that she will pick the exact date for her departure, probably no later than early February, as soon as she learns from the city's retirement board when she will be credited with 30 years' government service and thus become eligible for full retirement benefits.

Under McKenzie's three-year contract, which she signed in March and began July 1, the board contributed about $38,000 to her retirement account to make up for nine years when she worked for schools in Maryland.

As superintendent, McKenzie avoided the major controversies that marked the tenure of her predecessors in the past 25 years. She served longer than any D.C. superintendent since Carl Hansen, who resigned in 1967 after nine years in the post. Her predecessor was Vincent E. Reed.

McKenzie served almost two years in the Carter administration as deputy commissioner of the old U.S. Office of Education and as a deputy assistant secretary of the newly created U.S. Department of Education.

In a list of her accomplishments yesterday, McKenzie included the school system's computer literacy programs, its "public/private partnership" with business firms to create a range of special programs in city high schools, a dropout and truancy prevention campaign and the incentive programs, still relatively modest, "to recognize and reward" outstanding teachers.

"Until this year {we} were on a continuous achievement curve," with test scores improving for five years in a row, McKenzie said, "and we made the very difficult decision to change the test instrument and not leave it to a new superintendent."

With the new, harder tests, standardized achievement scores in reading and mathematics fell below national norms in all grades tested except for mathematics in the third and sixth grades. The elementary pupils' reading scores were just a few months below the national norms, but the averages for 11th graders were two years below the norm in reading and 1.8 years below the norm in mathematics.

In a successful appeal for more school funds last year, McKenzie said the elementary pupils' scores appeared to have "reached a plateau" and that secondary students were "not performing as they should."

A story in The Washington Post in June reported that in a private retreat this year, several members of the board had criticized McKenzie severely. McKenzie said in that interview, "No matter what I accomplish, they {the board} want more, more, more. And I guess that's what boards are for."

"It's been her style to go work behind the scenes to get what she wanted quietly," school board member Wanda Washburn (Ward 3) remarked yesterday. "I think sometimes the city likes a good fight, and they haven't seen one {involving schools} for a long while. We're dull, but isn't that better than the way it used to be?"