The Washington Teachers Union and the D.C. school system reached a tentative agreement yesterday on a three-year contract that would give moderate raises to the city's 6,000 teachers without requiring them to work a longer day.

The agreement, which was to be considered by the union's executive committee last night, is the result of more than four months of negotiations. It has yet to be approved by the school board and by a majority vote of the union membership.

Neither union President William Simons nor school system negotiator Kenneth Nickoles would release details of the agreement. But, Simons said, "I will tell you that the school day has not been increased."

Board members have said that the tentative contract includes a raise for beginning teachers from $19,100, the lowest starting salary among the large Washington area school systems, to about $21,500. That would still leave the District behind Fairfax and Montgomery counties.

With more than a third of its staff expected to retire in the next five years, the District is likely to have to recruit thousands of teachers. Board members have committed themselves to paying "comparable" salaries to those offered by competing suburban systems.

But board President Linda Cropp (Ward 4) said yesterday that "comparable" does not necessarily mean the highest in the area. And Cropp said that despite lobbying by Parents United, a District activist group, "if the board felt it was to the best advantage of the school system to have a longer day, we would negotiate it."

Rod Boggs, counsel to Parents United, said an agreement that neither catapults the District to the top of the local salary scale nor adds 30 minutes to the school day "doesn't provide comparable pay or comparable educational opportunities for the children."

But school board member R. David Hall (Ward 2) said Parents United "has tried to jump in the middle between the board and the union. The board has several issues that are more important than a longer day." Hall declined to say what those issues are.

Hall said the board had to negotiate carefully with Simons, who regained his position as union president in a bitter election last spring. "Bill Simons has to sell this to his membership and his membership has already shown a willingness to go to more radical leadership," Hall said. Simons defeated Harold Fisher, who had ousted Simons in the previous vote.

Simons had asked for a starting salary of $23,000. Montgomery and Fairfax counties pay $22,000 to new teachers.

Simons said he expected union members to endorse the agreement within two weeks. "I'm confident they will be satisfied," he said.