The Washington school system plans to send layoff notices to about 160 teachers to keep its work force in line with declining enrollment, officials said yesterday.

The first notices were mailed Monday to 100 of the school system's 2,467 elementary classroom teachers, according to personnel director Claudette Helms.

About 60 junior and senior high teachers will be sent notices within the next two weeks, Helms said.

Most of those receiving the notices probably will have jobs in the school system next fall, Helms said, as other teachers resign and retire over the summer. But she said the layoff notices are being sent in the spring to teachers with the least seniority in order to insure that the teaching force will be cut as required by the budget now awaiting action in Congress.

Betty Ann Kane, chairman of the D.C. Board of Education's finance committee, said the cutbacks are in line with falling enrollment, and will not result in larger classes. She said the board had wanted to keep the teaching force at its current strength in order to reduce average class size, but that the city government had rejected the request.

According to official estimates, D.C. school enrollment is expected to drop next fall to 115,100, about 5,000 less than this year and about 35,000 less than the peak it reached in 1969.

Just before fall classes started last September, the school system sent layoff notices to 70 teachers. Officials explained then that they had waited as long as possible in the hope of getting enough retirements and resignations to avoid layoffs entirely, but the action drew strong criticism.

"We don't want to catch that type of flak again," said deputy superintendent Edward G. Winner. "This time we're trying to do it as early as possible to give everybody as much notice as we possibly can so they can look for new jobs and we will have as much time as possible to place these people in the school system."

Helms said that all but about 10 of the teachers laid off last September eventually found other posts in the school system, but many of them lost several months' pay and most are in temporary slots outside their main field of teaching.

She said all the elementary teachers receiving layoff notices this week are probationary employes with less than two years experience in the school system.

Kane said the school board decided to keep all the special subject teachers in art, music, physical education and foreign languages, whose ranks were thinned by last year's cutbacks.

As a result, she said, children can expect to get classes in these subjects slightly more often than in the past because enrollment is falling. She said the number of reading, math and science specialists in elementary schools also will be kept the same, making it possible for them to spend more time with the students they serve.

Despite the overall cutback, Kane said, there will be an increase of 30 kindergarten teachers next fall, allowing all schools to have full-day kindergartens.