D.C. Superior Court Judge Gladys Kessler yesterday ordered striking Washington teachers to return to classes on Monday, but leaders of the Washington Teachers' Union said late last night that they will urge their members not to comply with the new back-to-work injunction.

The order by Kessler, issued yesterday morning, also reinstates for one week the union's old contract with the school board. The board decided to comply with the order even though it has opposed any extension of the pact.

Union President William H. Simons said after a 2 1/2-hour meeting of his executive committee that union leaders would urge continuation of the 19-day-old strike. The union membership, in its first formal meeting since the strike began, is scheduled to discuss the judge's order at 2 p.m. today at the Metropolitan A.M.E. Church, 1518 M St. NW.

Simons disclosed the executive commitee decision late last night as he left the Hotel Washington after more than a day of negotiations sponsored by Mayor Marion Barry.

Simons said the talks, which brought together school board members, union leaders and top city officials, were "an exercise in futility."

The union president refused to explain why the executive committee had recommended against complying with Kessler's order.

Since the strike began on March 6, the union has not retreated from its position that the teachers will not return to work unless the union's former contract with the school board is fully reinstated.

The board had let the contract expire in mid-February after extending it for more than a year. Board members have sought major changes in the pact that they contend gives the union too much power in running the schools.

School Board President Minnie S. Woodson described Kessler's new order as "sort of an interim peace that gives us a framework to really negotiate."

School Superintendent Vincent Reed said the school system would comply with the judge's order, including rein- stating the union dues check-off for a week, no matter what the union decides to do.

Kessler's new order was a preliminary injunction against the walkout. It replaced a temporary restraining order that expired yesterday. The new order goes considerably beyond the old one, barring strike activity by all school system employes, not just teachers.

In the order, Kessler required that round-the-clock negotiations on a long-term contract begin on Monday.

Late last night, federal mediators said such talkswould start on Monday at 10 a.m. at the headquarters of the federal mediation and concilation service, 2100 K st. nw.

They said the talks at the Hotel Washington, convened by Barry, had ended with little progress. As Barry left the hotel talks, which began at midnight Friday, he said, "Now I'll have to do some real deep thinking. I'll have to make a new assessment of my position on how to proceed."

Barry has threatened to seek City Council approval for emergency powers, allwoing him to supersede the school board and bargain directly with the union to end the strike.

Yesterday, Kessler sharply rejected the union's contention that the strike had been justified by unscrupulous labor relations on the part of the school board.

"The board did, by every available, legitimate and lawful means," Kessler said, "no more and no less what all participants in the collective bargaining process do: Try and squeeze the best possible bargain they can out of their opponent. But such bargaining activity does not constitute...fraudulent or dishonest...conduct."

On the other hand, she said, city teachers had widely flouted her no strike order even though they are expected "to serve as role models and examples of civic responsibility for their students to emulate."

In calling for the round-the-clock negotiations at the Hotel Washington, Barry said he feared that the school system's blue collar and secretarial employes might join the walkout soon.

The union which represents them, Council20 of the American Federation of State, County and Municipal Employees, is planning a membership meeting today to consider strike action. So far, its 4,000 members have continued to work despite the teachers' union picket lines.

Yesterday afternoon, Barry told reporters that he would not issue a statement urging compliance with Kessler's new order, despite a request by school board members that he do so.

"It's irrelevant what I think," Barry said. "They (the unions) have to do what the order says, but it ain't going to stop them either."

In her order, Kessler called for round-the-clock negotiations, similar to the session that Barry convened. She also required that school board members be on "one-half hour call."

She ordered that during negotiations "no adverse action" be taken by school officials against any techer, and added an injunction against anyone "coercing, intimidating, or threatening in any manner" any school system employes who come to work.

On Tuesday, Kessler will hold a hearing on a school board motion to begin collecting fines imposed against the union and its leaders for violating her earlier no-strike order. The fines, which escalate daily, already exceed $4000,000. No money has paid. CAPTION: Picture, Union leader William Simons announces meeting today to discuss judge's ruling. By Margaret Thomas-The Washington Post