The District of Columbia school board and the Washington Teachers Union reached what both groups termed a "memorandum of understanding" yesterday, averting a possible teachers' strike Tuesday.

The tentative agreement extends for a third time the union contract that expired a year ago. It also calls for negotiations until Feb. 14 and says that both sides agree to request by Wednesday the help of a federal mediator.

Both sides noted yesterday that, although next week's strike threat has been averted, there is nothing in the tentative agreement that rules out a possible future strike. They also said little progress has been made in negotiations in recent months.

The "memorandum of understanding" was signed yesterday by union president William Simons and is to be presented to the school board at a meeting Tuesday. The memorandum was formulated by the board Thursday night and is expected to gain swift approval.

The contract had been extended last January for six months, but negotiations proved fruitless and no contract existed between July and October when a 90-day extension was negotiated. That extension would have ended Tuesday, and Simons had threatened a strike.

The possibility of a strike could recur by mid-February because the tentative agreement does not provide for an extension beyond Feb. 14 if mediation is unsuccessful.

According to board rules, if negotiations involving a mediator remain deadlocked after 15 days, the mediator must begin a fact-finding proceeding. In fact-finding, a third party sifts the proposals, determines the merits of each case and recommends an equitable settlement that is not binding.

Board rules set no time limit on fact-finding, according to a source familiar with the procedure. If the fact-finding continues beyond Feb. 14, the union would be in position to demand another extension while still threatening a strike.

As of yesterday, several major issues -- such as how to evaluate teachers, the length of the school day and year and how teachers can discipline students -- have not been discussed, both sides acknowledged.

"When the school superintendent says that Mr. Simons hasn't been at the bargaining table, he is right,"union president Simons said. "Mr. Simons has not been at the bargaining table. With that kind of garbage, with their negotiators refusing to talk about issues, to negotiate, Mr. Simons has more valuable things to do with his time than to sit there and listen to all that garbage...

"Nothing has been accomplished," Simons said.

According to board members and negotiators on both sides, the board's stance has been to refuse to negotiate several issues that board members describe as "policy matters."

According to members of the 11-person board, school system officials generally feel that school policy should be decided by the board or system administrators without input from teachers or the union. When negotiations began, then-board president Conrad Smith said the board would try to recapture some of the policymaking authority gained by the union in previous contracts.

The unions bargaining position has been to try to maintain its role and that of the teachers in school policy decisions. Simons has said that the current contract is a good one and that he is trying to improve it by spelling out conditions in the classroom. That conflicts directly with the board's stand against letting teachers or the union influence policy decisions.

"The board wants teachers out of everything that has to do with school policy," Simons said. "You've seen the board in action. Are they in control? As a matter of course, the board is not involved in the day-to-day operations of the schools. How are they going to be able to decide policy without consultation and discussion with the people in the pits, so to speak?"