They say they will remember Fairfax County Deputy School Superintendent Donald Lacey for many things, but particularly for the way he moves quickly through a school, noticing clean floors or smudged walls, noisy classrooms or silent libraries.

"He strides so quickly through a building that anybody with short legs runs to keep up," said Delores Bohen, a school official who worked for several years as Lacey's special assistant. "You're convinced he's just taking a cursory tour, but you later find out he can tell you all the details."

Lacey, 55, who is renowned for his ability to absorb details and is reputed to be a frank and decisive manager, has announced he will retire after 27 years with the Fairfax system.

His retirement May 31 is important because it comes at the same time that his boss, Superintendent William J. Burkholder, is leaving the huge Fairfax system.

"I'm sure the system will carry on," said John Scott, principal of the Little Run Elementary School, echoing the views of many, "but it's very hard to lose all your biggies at once."

Burkholder, who resigned last December admid a mounting furor over a proposed $157,000 -a-year pay package, will be replaced by Robert R. (Bud) Spillane, head of Boston's school system. Lacey's $73,175-a-year job is expected to be divided between two successors, neither of whom have been selected.

"I can grant you that there are any number of exceptionally talented people in this school system that can come in and do my job," Lacey said in an interview. "I'm sure any loss of Jack and Don will be only momentary. There will be new visions, and greater visions."

Lacey, who did not seek Burkholder's job, said it took him several months to decide to retire. "It was important for me, at 55, to take a look at where I was going to go from here," he said.

Although he once planned to leave the system in March, Lacey said he decided to stay until after the School Board had picked a new superintendent. He said he may make a second career in real estate management.

A former seventh grade teacher and elementary school principal, Lacey was an area superintendent in Fairfax before Burkholder nominated him as the system's deputy superintendent in September 1982.

He worked well with Burkholder, but of the two, Lacey was perceived as the tough guy, associates said. "In business, in government or in a school system, the chief executive officer has to have an alter ego," said former School Board member Toni Carney, "and that person is usually someone who can think similarly to the chief, but who can be the heavy. This is the role that Don has played."

"If you did the best you could, that's all he wanted," said Lillie G. Morarity, who started as a school maid and rose, with Lacey's encouragement, to become coordinator of school plant operations. "Once he knew you were working for him, you had a real friend in the world."

As deputy superintendent, Lacey also gained a reputation as a stickler for building cleanliness and efficient management, officials said, and was exceptionally straightforward.

"I think uniformly what you hear from people is that you know where you stand with him, and this is very important to people," said Lenore Plissner, president of the Fairfax Association of Elementary School Principals, and a former School Board member.

He possessed a keen memory, too, and could recall the smallest of personal details: a pending knee operation, or someone's sick aunt, associates said. "He's a guy who takes education -- How can I put it? -- intensely," said Little Run's Scott. "He really lives it. I think he's got a big fan club."