Almost 40 percent of the District's public school teachers and nearly half of their students were absent yesterday as the city schools remained open on Good Friday for the first time in memory.

Superintendent Floretta D. McKenzie decided Thursday that classes would be dismissed at 12:30 p.m. because of fears of massive absenteeism by teachers.

The Washington Teachers Union, which had objected to dropping the traditional holiday and had encouraged teachers to take the day off for religious observances, had estimated that about 80 percent of the District's 5,200 teachers would be absent.

Both school and union officials speculated that the number of absent teachers had been reduced by McKenzie's decision to close school early. The attendance figures were provided by Janis Cromer, the superintendent's spokeswoman.

"I think the concerted action by the teachers has certainly indicated to the school board how sensitive this matter is," union President Harold Fisher Jr. said yesterday. "I do not think they will vote not to include Good Friday in the spring break next year. If they do, I will be very surprised."

School board President R. David Hall could not be reached for comment yesterday. Earlier he had urged that teachers be given the day off, but the move was blocked at a meeting Thursday when the 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.

The union has filed charges of unfair labor practices against the school board for changing the traditional school calendar without negotiating with the union or consulting it. The changes also included having teachers start work in late August, a few days before Labor Day, so that classes can end earlier than usual in June. The board said it had approved the changes, which McKenzie did not recommend, to make it easier for students to get summer jobs.

Fisher said the D.C. Public Employee Relations Board is expected to rule on the union charges soon.

Teachers who were off yesterday took a day of personal leave, which is allowed under the union contract. But Fisher said the school board should have granted administrative leave "in the interest of religious freedom" and not charged the time against the three days of personal leave a year that each teacher is allowed.

Cromer said yesterday's half-empty schools reported no major problems as substitutes and parent volunteers filled some of the gap left by the absent teachers, but she acknowledged there was far less teaching than usual.

For example, at Francis Junior High School at 24th and N streets NW, most students spent the morning watching films in the auditorium or playing soccer outdoors. "Because of the lack of staff, some students filtered out on their own," said Principal Gary Geiger. "It just wasn't possible to run our normal classes."