The Washington school system plans to transfer 48 elementary teachers to its junior high schools next fall in a effort to raise student achievement and save teaching jobs as enrollment continues to tumble.

"Many of these junior high kids are so far behind, it's like teaching fourth and fifth grade," said associated superintendent James T. Guines. "You have to teach the basic skills of reading and math, and many of the subject-matter teachers in the junior high schools just don't know how to do that well."

Under the plan announced yesterday, the 48 elementary teachers placed in junior highs will be called "transition" teachers.

Ronald Webb, an assistant to Superinedent Vincent Reed, said they will teach small classes of seventh graders-with no more than 15 students in each section. The classes will cover remedial work in mathematics and communications skills, Webb said, and will start wherever a student's skill level is.

Webb said the students assigned to the classes will be those with the most serious achievement problems, as shown by tests given this spring.

In an effort to prevent D.C. youngsters from slipping behind in academic work. Reed announced that the school system would expand its pre-kindergarten program next fall so that four-year-olds throughout the city will be able to attend free all-day classes.

This year the school system provides full-day classes for about 1,300 four-year-olds and and half-day classes for another 1,500.

The school system also has full-day kindergarten classes for the first time this year for all five-year-olds-about 6,400 youngsters-who want to attend them.

Washington is the only public school system in the area offering full-day kindergartens on a wide-scale. Next year it will be the only system offering full-day pre-kindergartens.

"The superintendent feels very strongly," Webb said, "that early childhood education is really crucial. The earlier we can get children in school with a full academic program, the better we will be academically."

Webb said Reed was out of the city yesterday and could not be reached for comment.

He said no approval is needed by either the school board or City Council because both the reassignment of elementary teachers to junior highs and the all-day pre-kindergartens can be carried out with teaching staff already included in the school budget.

Next year the school system expects to have about 6,500 teachers, just as it does currently, even though enrollment is expected to drop about 6,000 below the 113,000 students now attending.

The new all-day pre-kindergarten classes will require 47 more teachers than the 100 who teach pre-kindergarten this year, Webb said.

He said he hopes the all-day classes will be attractive to working parents and will persuade some of them to switch their children from private nurseries to the public schools.

Even though the school day runs from 9 a.m. to 3 p.m., Webb said it is not tiring for most 4-year-olds because it includes rest periods.

"In today's world of working parents many children are put into day-care group situations at two and three years old," Webb added. "By the time we get them, they're used to it."

Over the past five years, Washington first graders have made considerable gains on reading and math achievement tests and now score as high as national norms. School officials say the progress is accounted for by the school system's extensive program for young children.

Slight gain have been recorded by D.C. third graders, though they still lag considerably behind average achievement nationwide.

But older students in the school system have shown no gains on average from the very low levels reached around 1970. Last year D.C. ninth graders lagged three years behind the national norms in both reading and math, indicating that their average level of achievement was the same as that of sixth graders throughout the country.

Guines said the large achievement deficit has caused serious problems for junior high teachers, who are trained as specialists in their own subjects such as history, science, or English and not in teaching basic skills. He said the elementary teachers being transfered to junior highs have the training to teach skills many students lack.

Even though Supt. Reed has said he wants to do away with "social promotion" the school system has not yet established any definite list of skills the students must master before they can move from elementary schools to junior high.

"We're still working on that," Webb said yesterday.