The District of Columbia school board has directed Superintendent Vincent E. Reed to stop deducting union dues from teachers' paychecks, a move apparently calculated to pressure the city's teachers' union to agree to a new contract.

Virtually all major issues in the prolonged negotiations, such as lengthening the school day and school year, are still unsettled, sources said. Negotiations were suspended this week because of the snowstorm but are scheduled to resume today.

Washington Teachers' Union President William H. Simons has previously threatened to call a strike if the board ceased deducting union dues from the paychecks of teachers.

"No one wants to precipitate a strike," said board vice president Carol Schwartz. "But the contract has expired, the union knows that and we know that. And when a contract expires all its provisions expire including any that call for dues check-offs. With the expiration of the contract that (the dues check-off on paychecks) expired too."

Simons refused to come to the phone yesterday to speak with a reporter and through his secretary said he would have no comment on the board's action.

The contract between the union and the board orginally expired in January 1978 but has been extended three times by the school board, once under threat of a strike by the union. But when the last extension of the contract ended Feb. 14 the board took no action to again continue the contract despite warnings by Simons that he would not negotiate without a contract and would strike.

The union, however, continued to negotiate past the Feb. 14 deadline and Simons said they would continue the talks if all provisions of the contract, particularly the automatic union dues check-off on teachers' paychecks in which about $59,000 a month is collected, were continued.

George Margolies, legal counsel to Superintendent Reed, said the superintendent had delayed asking the city agency that handles teachers' paychecks to end the dues deductions on teachers' paychecks because he felt progress was being made in negotiations in the last week of negotiations.

But Margolies said the board told Reed to take action to end the dues check-off immediately.

"We have no authority to take money out of anyone's paychecks, for union dues or anything else," said Margolies. "Without the contract we have no authority to take the money out for union dues. So we moved to nitify the D.C. finance office.

"It was as simple as that," he added. "There's nothing to tell the union about. We both agreed the contract would end last week and they can read and we can read."

The board asked Reed to take steps to end the automatic dues deductions at a closed briefing on the status of negotiations with the union Wednesday night. At the same meeting Reed told the board, according to board members, that he plans to contact suburban school districts to obtain their list of substitute teachers in an effort to keep city schools open if a strike does occur.

And Reed told the board that he plans to contact city officials to insure that teachers who do not want to stay out during a strike are able to get into schools without harassment by picketing teachers.

"We've heard that a lot of people didn't want to go on strike last time" said Schwartz. "There was a lot of intimidation and shenanigans to keep teachers from crossing the picket line. We want to give teachers who want to work the chance to work if a strike comes."

According to persons close to the negotiations Simons asked the board's negotiators on the day after the last contract extension expired if the school system would be sending principals a memorandum telling them that the contract was void. The board's negotiators said they would not send the memo out the next day by made no other promises.