Negotiators for the District of Columbia school board and the striking Washington Teachers' Union remained in sharp disagreement last night in their efforts to end the teachers' two-week-old walkout.

After five hours of negotiations that ended early yesterday evening, it appeared uncertain whether an agreement could be reached in time for teachers to return to work Monday. Another bargaining session is scheduled for 1:30 pm. today.

The school board's chief negotiator, Kenneth W. Nickoles, said that "progress has been made" and that the Board of Education had made "substantial" changes in its bargaining position. He expressed hope that an agreement to end the strike could be reached today so that the walkout would be over in time for Monday's classes.

However, union president William H. Simons emerged from yesterday's talks angrily denouncing the school board negotiators for what he termed "going backwards." He said the board was presenting "garbage" proposals which amounted to "union busting." Simons, who had previously voiced optimism that the strike might end by Monday, expressed extreme doubt last night that a settlement was in sight.

"I doubt it in any way, shape or form," Simons said, when asked whether teachers were likely to go back to work on Monday.

At yesterday's session, according to both school and union officials, board negotiators offered a new proposal for an "interim agreement" designed to end the walkout, which began March 6.

The temporary accord would permit negotiations to resume on a long-term union contract. The union's former contract was canceled by the board last month after three earlier extensions. The cancellation led to the teachers' strike.

The union plans to make a counter offer today, according to negotiators for both sides.

Although board negotiator Nickoles characterized yesterday's board proposal as a significant step toward a possible settlement, union president Simons asserted that the offer represented "no substantial changes" and "no progress."

Simons specifically objected to provisions in the board's proposal that he said would give the school board an "exclusive right" to assign teachers to nonteaching duties -- a power that Simons said the board currently does not have.

Nickoles declined to discuss details of the board's proposal, but a copy of the school board offer was made public by Simons.

The board's offer would provide for the 'i'nterim agreement" to remain in effect until June 15. It appeared unclear what would happen after June 45 if a new contract had not been negotiated.

Under the board's proposal, the interim agreement would reinstate the union's grievance and arbitration procedures and apparently also would reinstitute the checkoff -- automatic payroll deduction of union dues -- a provision ended by the school board last month. The checkoff is a major source of union founds, providing it with about $70,000 a month, according to school officials.

School board negotiators offered to abandon their efforts to lengthen the school day in exchange for an agreement by the union to give up former contractual guarantees for teacher "planning periods" at the start of the school day. The proposal does not specifically mention the board's plan to lengthen the school year -- a separate issue from the proposed school day extension. The school year apparently would remain a subject of negotiation.

The board's proposal states that the school board would be granted an "exclusive right to schedule employes in the bargaining unit during the current work day." Simons denounced the provision an a way of allowing teachers to be assigned to nonteaching duties.

Under the terms of the "interim agreement" proposed by the board, the union would be required to keep this provision in its next long-term contract, whenever such a contract is signed.

All other points of contention would be submitted to further bargaining sessions and possibly to fact-finding panels after teachers return to work, according to the board proposal.These negotiations would be aimed at concluding a new long-term union contract.

As a result of the strike, classroom instruction has been disrupted at most of the city's 190 public schools. According to school officials, only about half the city's 113,000 public school students have attended school daily.

School and union officials have given widely differing estimates of the number of teachers participating in the walkout. The union, which represents about 5,200 of the city's more than 6,000 teachers, has said that more than 80 percent of the teachers have stayed off their jobs. School officials estimated that nearly half of the teachers were at work last week.

Meanwhile, fines against the union for violating a no-strike order by D.C. Superior Court Judge Gladys Kessler have continued to accumulate. They will rise to $114,750 by Monday, but so far the union hasn't been required to start paying.

Simons said that if teachers do not return to work on Monday, the strike "is going to escalate," apparently meaning he expects active support from other unions.

Yesterday, school superintendent Vincent Reed said that the nine days of school disrupted by the strike and school closings for heavy snowfalls had caused the system to make plans to hold summer classes for students who fail regular courses or who want to take advanced work.

Because of the four days lost to snowstorms this winter, Reed said spring vacation will be trimmed to two days off -- April 12 and 13.

Summer school has not been held in Washington for the past two years because of budget cutbacks.

Reed said that he expected that summer school could be financed without now asking for extra city money because of savings produced by the strike and cost-cutting measures taken earlier.

The superintendent said planning for summer school, which probably will cost about $500,000, started before the strike began but has been "intensified" because students now are losing instruction at a crucial time in the school year.

"It's going to be the full-fledged kind of summer school we always should have," Reed said. "We really need it now because the strike is hurting the children.... The union leadership doesn't care about the kids."

Reed said that he is also exploring the possibility of adding extra time to the school day for make-up classes when the strike is settled and giving extra pay to the teachers who work extended hours.

The superintendent said that he has received more than a dozen reports of nonstriking teachers having tires slashed on their automobiles and other types of vandalism and intimidation.

At one school, he said, paint was poured on the cars of teachers working inside. Elsewhere, he said, a working teacher was threatened by two strikers as she left home and parents who volunteered to fill in for striking teachers have been told their children "won't be taught" when the strikers return to their classrooms. return to their classrooms.

Reed said he is collecting written statements on all complaints.

"There's going to be due process," Reed said. "But I'm to deal with this sort of intimidation as severely as we can. It's been nasty."