For the first time in at least a decade the Washington school system sent layoff notices yesterday to 70 tenured teachers who will lose their jobs Sept. 30 because of declining enrollment and a decrease in the number of teaching positions.

Claudette Helms, the school system's director of personnel, said the cutbacks would have the most impact among specialist who teach foreign languages and music in elementary schools. But she said there also would be substantial layoffs among aret and physical education teachers.

Helms said school officials delayed sending the notices as long as possible in hopes that enough teachers would retire to avoid any layoffs. But officials of the Washington Teachers Union said the fired teachers should have been told they would lose their jobs last spring so they could have had time to look for new ones.

"You cannot just go out in September and find another teaching job," Harold Fisher, the union's chief field representative, said yesterday. "Some of the teachers just found out they wouldn't have job when they reported to work today, and some got phone calls last night."

Yesterday was the start of fall term planning and orientation meeting for D.C. school teachers. Pupils will start classes Wednesday.

The school system is expecting about a 3 per cent decline in enrollment this year and its budget contains a similar drop in teaching staff - about 220 of the 7,300 teachers employed last year.

Officials said that only about 150 teachers retired or resigned over the summer even though those who retired by Tuesday were eligible for a 5.9 per cent bonus in pension benefits.

Helms said a few layoff notices might be rescinded if more teachers retire, but added she was not hopeful that many of the laid off teachers would get a reprieve. In addition, whe said, layoff notices will be sent during the next two weeks to a group of teacher aides, but she said she did not know how many would be affected.

Over the past four years, the number of teachers in the Washington school system has declined by about 10 per cent, as enrollment has plummeted even more sharply. However, until this year all the reductions, Helms said, were made through attrition or by dropping temporary teachers who had year-to-year contracts.

This fall, she said, the system will have virtually no temporary teachers. As recently as three years ago, there were as many as 800.

Helms said the school system's problems this fall were compounded by the ending of the Response to Educational Needs Project (RENP), a federally funded program in Anacostia that had 104 employees last spring. She said all of them have been given regular jobs in the school system, but this placed in Jeopardy the jobs of other teachers with less seniority.

Under federal Civil Service rules that govern reduction-in-force layoffs throughout the D.C. government, teachers with the least experience in each job category are the first to be laid off.

School officials said the special teachers jobs were picked for cutting, rather than regular classroom teaching slots, in order to avoid increasing average class size for students.

The cutbacks spell the virtual end of the elementary foreign language program, and severely curtail the elementary programs in art and music. Overall, 51 elementary school teachers are being laid off, 19 in secondary schools.

All employees of the school system were notified in general letters this spring and summer that the school system might suffer a reduction in force. But none was told until yesterday that his or her job was involved.