The D.C. School Board tried yesterday to avert a threatened teachers' strike by offering the Washington Teachers Union two proposals concerning the number of hours that teachers on the contract negotiating team may leave their classrooms for bargaining sessions.

But leaders of the teachers union rejected both proposals and stood by the decision of their executive committee to call a strike beginning tomorrow. The teachers, who already have been under a union "work-to-rule" order not to attend any school activities beyond the official school day, voted last Tuesday to give the union authority to call a strike.

The main issue in the dispute is the amount of time that the eight teachers who are members of the union's negotiating team can be away from their classroom for contract talks. The union is asking that the teachers have unlimited time, at full pay to leave their classes for the negotiations.

Also at issue is reinstatement of the teachers' old contract, which expired July 25. The teachers union wants the contract extended before negotiations on a new contract begin.

Yesterday, in an emergency session called because of the strike threat, the school board repeated an earlier offer to extend the expired contract for 90 days if the union accepts an 80-hour or 12-day limit on the amount of time teachers on the union negotiating team can be away from their classes for contract talks.

In a new proposal to the union, the board also offered to "begin negotiations immediately on the substantive issues of a new contract without limitations on negotiations during school hours" (for teachers on the union's team of negotiators.)

But Harold Fisher, assistant to the president of the teacher's union, said last night that the board's offer to give teachers on the negotiating team unlimited time out of the classroom for bargaining is unacceptable because it does not include reinstatement of the expired contract.

Fisher said reinstatement of the contract on the condition that the union accept the 80-hour or 12-day limit for teacher-negotiators also is unacceptable.

"Without the contract in force our people are without protection," Fisher said. "We want teachers to have their rights protected while the new contract is discussed."

Fisher also said there was no limitation on the amount of time the teacher-negotiators could leave their classroom for contract talks under the contract that expired July 25.

"Our position is that the board already agreed to unlimited hours for the negotiators in the contract and we still stand by that," he said. "It can only change if we bargain on the issue in the contract talks with the board."

At yesterday's meeting of the school board, vice president Carol Schwartz said the board's new offer to the union was proof that the board is not stalling the start of negotiations, as the teachers union has charged.

"It is important that the community knows the major issues," Schwartz said in the midst of an intramural battle between members of the school board over what kind of proposal the board should send to the union.

"The board of Education is not trying to call a strike," she said. "Our teachers union is calling a strike. The strike call is an effective bargaining tactic but it is not the intention of this board to roll over and play dead. You should know that the last contract negotiations went on for 2 1/2 years. I think we would agree that that is an abnormally long time, and the students, the parents and this school board paid for that."

The school board voted 9 to 1, with member Frank Shaffer-Corona in opposition and John Warren abstaining, to adopt the two proposals it sent to the teachers union yesterday.

Superintendent Vincent Reed, who attended the school board meeting, told Warren that, although only eight teachers-negotiators are at the heart of the issue that may cause a teachers' strike, the school system must be concerned about every student in those teachers' classes.

After the school board's action, several board members said the dispute is a "preliminary skirmish," between the union and the board over several policy issues that will be negotiated in the next contract.Among those policy issues are the length of the school day, testing teachers for their qualifications, union dues check-off on paychecks, grievance procedures and the terms of maternity leave.