D.C. School Superintendent Floretta D. McKenzie, faced with the prospect of massive absenteeism by teachers, decided yesterday to close the city's public schools today at 12:30 p.m. in observance of Good Friday.

The decision was announced in the early afternoon after the D.C. school board failed to muster enough votes to reconsider the decision it made last year to start spring vacation on the Saturday before Easter instead of following the custom of closing for Good Friday.

McKenzie told the board she expected at least 40 percent of the city's 5,200 teachers to miss school on Good Friday by taking a day of personal leave, which is allowed under the teachers union contract.

Washington Teachers Union President Harold Fisher Jr. said he expected about 80 percent of the teachers to take the day off.

The mass absenteeism, McKenzie said, put the schools in a "crisis situation." She told the board that principals were trying to arrange for substitute teachers, but said that in many schools classes might double up or students might be kept in the auditorium watching films.

George Margolies, the school system's legal counsel, said McKenzie acted under a provision of the board rules, normally used for snow emergencies, permitting the superintendent to close schools because of "adverse weather conditions or other emergency conditions affecting the health, safety or welfare of students."

Janis Cromer, the superintendent's spokeswoman, said the rule previously had been used only during bad weather. But she said the early closing was necessary because of the expected shortage of teachers and had been sanctioned informally by a majority of board members.

She said principals and other administrative employes will work a full day.

Elsewhere in the area, Prince George's and Montgomery county public schools are closed today for students and all employes. Schools are open in Alexandria and Arlington. In Fairfax County students have the day off but it is a work day for teachers. Officials in the three Virginia school systems said they did not anticipate a large number of absent teachers.

Last spring the District school board voted 7 to 3 to keep schools open on Good Friday as part of a plan to alter the school calendar here so that classes can end a week earlier than usual in June to make it easier for students to get summer jobs.

Yesterday's effort to get board approval to close for Good Friday was blocked by member Eugene Kinlow, who refused to go along with six other members present in waiving the board rules to consider the issue at the special meeting, which had been called to take up several budget issues.

Under the rules seven of the 10 board members -- a two-thirds majority -- must approve a waiver. Despite a plea by board President David Hall, Kinlow said there was "no good reason to second-guess our decision" to keep schools open on Good Friday.

Cromer said that over the last two weeks, school officials have received complaints from parents and teachers that having school on Good Friday would interfere with vacation plans or with attending church services, which traditionally start at noon. After the half-day closing was announced, some parents complained that the sudden schedule change would cause child-care problems for the large number of the District's 87,000 students who come from homes where both parents work.