Washington public schools will open an hour early and close an hour late for eight weeks starting April 16 in a move to help students make up work lost during the city's 23-day teachers' strike, Superintendent Vincent Reed announced yesterday.

Reed said participation in the early morning and late afternoon classes would be voluntary. But officials estimated that about a third of the school system's 113,000 students would take part.

In addition, Reed announced that class students in most elementary schools would be rearranged to provide extra time for teaching of reading, writing and mathematics, with less time than usual being spent on other subjects.

For junior and senior high school students, Reed said there will be special tutorial classes, limited to 15 pupils each. Reed said special tutors, including nonprofessional aides and possibly retired teachers, would be hired to help regular teachers run the tutorial classes.

The plan does not include extending the school year beyound June 15, its scheduled last day. School officials said they could not estimate th cost of the expanded-hours program.

Reed's plan was distributed after a special school board meeting, at which the board voted to comply substantially with a court order that preceded th end of the strike last Thursday.

Under the order, issued by D.C. Superior Court Judge Gladys Kessler, the teachers' union's old contract was reinstated until July 15 while efforts continue to reach a new agreement.

The board voted 6 to 4 to extend the pact to June 30. School Board President Minnie Woodson said the earlier date was selected in an effort to indicate that the board thought it was not bound by the judge's decision and also in hopes that a new contract might be ratified by teachers soon after the spring term ends.

The board voted unanimously that it would take "no act of harrassment or adverse action" against individual teachers for their "mere participation" in the walkout.

Woodson said she thought this "clarified" Kessler's order, which declares that "no adverse action" can be taken against teachers until July 15.

The board wants to indicate, Woodson added, that its promise to refrain from disciplining teachers only applies to striking itself. She said teachers who may have engaged in intimidation or vandalism or violate work rules this spring would still be subject to normal disciplinary procedures.

Teachers' union officials could not be reached for comment last night.

Several days ago union lawyers William B. Peer said Kessler's order was "self-executing." He indicated that the union would accuse the board of contempt of court if it did anything to violate the order.

Even though all city schools remained open throughout the strike, Reed said the walkout had a "devastating" effect on instruction during one of the most important parts of the year. For most of the 17 school days that fell during the strike, only about half the students turned up for class.

According to school board figures, about half the city's more than 6,000 teachers stayed away from work throughout the strike. The union said about three-quarters of the teachers took part in the walkout.

Yesterday, Vice Superintendent Elizabeth C. Yancey said teachers would be chosen by principals to teach the extra classes solely on the basis of their subjects and skills. She said strict orders had been issued that no preference would be given to teachers who took part in the strike or to those who continued working.

During the last week of the strike, the teachers' union asked that any extra work be given to those who lost pay during the walkout.

Yancey said those teaching beyond regular school hours would be paid about $13 an hour.

In addition to the major changes in school programs this spring, Reed said the school system would operate summer school for remedial work and advanced courses for the first time in three years. The summer session will run six weeks in July and August, but officials said detailed plans for it have not yet been developed.

The special classes this spring will concentrate on test-taking as well as basic skills, Reed said. Because of the strike, one set of systemwide tests has been delayed from mid-April to late-May. Standardized test in reading and mathematics will be given as scheduled from May 7 to 11.

Reed also extended until Wednesday the third marking period of the school year, which had been set to end today. He said teachers should give grades in subjects all without "in completes" but should not penalize students for work they didn't do during the strike.