With the Washington Teachers Union planning to ask its members to authorize a strike, negotiations will resume today between the union and the District of Columbia school board on terms for a new contract.

Deliberations recessed at 1 a.m. yesterday after a 16-hour session, with federal mediator John A. Wagner reporting slow progress. It was the only negotiating meeting of the week, a result of bad weather.

"We are in some very sensitive issues and it is going to take total concentration of both parties to keep the thing moving along," Wagner said, adding: "I feel optimistic always."

The contract between the teachers' union and the board originally expired in January 1978 but was extended by the board three times, finally ending Feb. 14. The board, apparently trying to pressure the union to settle, then ordered an end to the deduction of union dues from teachers' paychecks.

In response, William H. Simons, the union president, said the labor group's executive board voted yesterday to ask the 5,000 members to authorize a strike at a time to be set by the board. Simons refused to speculate if and when a strike may come.

Although there are "a number of major items" still unsettled, Simons said he was "optimistic and hopeful that we are going to be able to make some significant movement" to avert a walkout.

The union membership will be held at 7:30 p.m. tomorrow at Greater New Hope Baptist Church, 8th and I streets NW.

Staff officials of the school system, who are representing the school board in the negotiations, could not be reached for comment.

Negotiations mainly involve working conditions and personnel practices, but not salaries, which by law are set by the D.C. City Council.

Among unresolved issues are the school board's proposed lengthening of the school year and school day. In a tape-recorded telephone message to members yesterday, Simons said the board "wants to remove... the control of pupil grades by the teacher."

Minnie S. Woodson, school board president, called that an over-simplification. The board, she said, feels that anything to do with grades is not a matter of working conditions of teachers that belongs in a labor contract.

"The points (at issue) are educational policy that should definitely be in the hands of the Board of Education and in the hands of the superintendent, as delegated," Woodson said. "The pupil is not to be a pawn in the union contract."

Simons said the union "simply does not know what they (the school board members) want" on the grades issue. "The question I would like to see posed to the Board of Education is just what it is in the (old) union contract that prevents the board from carrying out its (policy) function," he said.

Today's negotiations, closed to the public, will be resumed at 1:30 p.m. in Payne Elementary School, 15th and C streets SE.