More than 240 District teachers are absent from their classrooms each week, leaving hundreds of students without instructors because substitutes are seldom hired, according to a study released recently by the school system.

The absences, which represent about 6 percent of the city's 3,765 teachers, occur at higher rates on Mondays and Fridays and in the city's junior and senior high schools, according to the study, which was ordered by the school board to determine the amount of time "wasted" because of teacher absenteeism and extracurricular activities that infringe on the school day.

Usually no substitutes are assigned to replace absent teachers, according to the study, which surveyed lost instruction time in 12 of the city's 165 schools between December 1984 and last spring.

A spokeswoman for the public schools said the absentee rates were not unusually high. Harold Fisher, president of the Washington Teachers Union, attributed the absences to job-related pressures created by overcrowded classrooms and "insensitive" administrators.

Janis Cromer, the school system's spokeswoman, said, "In most organizations where you have absences, they are going to be on those days Mondays and Fridays and from our review there doesn't appear to be an inordinate amount of absences. Teachers earn 10 days of sick leave a year and use, on average, about seven days a year."

But school board member Edna Frazier-Cromwell, who requested the study, disagreed: "Teacher absenteeism was much higher than I expected," she said, describing the findings as "disturbing."

"There is a pattern of absenteeism . . . . Taking Mondays or Fridays off allows you to get extra days in and stretch your weekends," she said.

The study, the first of its kind conducted in District schools, is the beginning of ring with some of his work. students when teachers are absent. Each school appears to cope with the problem differently. Some schools divide students among other teachers, while others send pupils to see educational films in the auditorium.

The survey reported that junior and senior high school students are hardest hit by teacher absences because a single teacher may have a series of classes containing 30 or more students.

In elementary schools, a single teacher usually has only one class of about 26 students, school records show, so the absences affect fewer students.

Elementary teachers have an absenteeism rate of 5.5 percent on Mondays and Fridays and, on average, 4 percent of them are absent each week. Secondary teachers have an absenteeism rate of 5.6 to 7.6 percent on Mondays and Fridays and, on average, 6 percent of the senior high and 5.2 percent of the junior high school teachers are reported absent each week, according to the study.

Fisher of the teachers union said many instructors are absent because "teaching is a high-stress and demanding job. In any such job, you would expect that there's going to be a tendency towards absenteeism. The union is certain that we are dealing with stress and not just people who just don't want to work."

He said teacher absenteeism can be reduced through a new program created by the union and the school system "to deal with what we already perceive as a high-stress problem for teachers."

Under the program, teachers receive treatment for a wide range of psychological and medical problems, such as depression, anxiety and alcoholism, he said.

Frazier-Cromwell said she had asked for "a follow-up on the study" to determine the major reasons for absences and to help school board members devise a strategy to curb teacher absenteeism.

The survey also compiled information from teachers and principals concerning the amount of class time lost to allow students of skep classes to attend athletic events, pep rallies, end-of-school activities and seasonal celebrations, such as school-sponsored Christmas programs.

Frazier-Cromwell said, "Students were taken out of class early to get transportation to athletic games. That disturbed me alot."