At Thomson Elementary School a secretary was sorting a tall pile of folders. At Francis Junior High, the principal was tuning a new color TV.

Yesterday was the fifth day before the scheduled return to work of Washington's public school teachers. Preparations for the fall term were well under way.

Whether the term will begin normally, though still is uncertain because the school board and the Washington Teachers Union have not agreed on a new contract. Without a contract, the union has threatened a strike.

"We're going ahead normally," said Gary Geiger, the principal at Francis, 24th and N Streets NW. "And we're praying it's all settled before we're supposed to start."

"I think it's going to be the best year we've had so far," Geiger said, and then he stumbled "if . . . if they don't strike again." Teachers are scheduled to report to work on Tuesday. Classes begin Monday.

Since Aug. 20, union president William Simons, who led a 23-day strike last March, has met eight times with two school board negotiators, trying to reach agreement on a new pact.

Both sides say differences have been narrowed, but no bargain has been struck yet.

After yesterday's hour-long session at school board headquarters, Simons said he felt "guarded optimism," and would take part in more negotiations today.

Kenneth W. Nickoles the school board's cheif negotiator, said Simons "backed off" yesterday from a "promising proposal" the union leader made the day before. "I thought we might be able to wrap it up" Nickoles said, "but we're not there yet."

The latest round of negotiations is centered on recommendations made Aug. 8 by James M. Harkless, a fact-finder appointed after the March strike.

The strike ended after a D.C. Superior Court judge met a key union demand and reinstated the union's old contract with the school board until mid-summer.

The board had been seeking major changes in the pact, signed in 1976, which it said gave too much power to the union and required teachers to spend too little time in the classroom. The union countered that the board was trying to "break" it and take away gains teachers had previously won.

In his report, Harkless recommended that the union accept board proposals dealing with teacher evaluations, student grading, hall duty and record keeping.

But on three major issues -- extending the school day, lengthening the school year, and restricting grievances that could go to arbitration -- Harkless sided with the union.

On Aug. 13, the board voted unanimously to accept Harless's proposals, though it said any new contract should contain a clause allowing the issue of work time to be reopened as soon as the board gains authority to set wages on Jan. 1.

The union praised the board action as a "positive step." But when negotiations resumed, it suggested 35 changes in Harkless's proposed contract language.

It also sought board support for reducing to $65,000 the $343,000 fine imposed on the union by D.C. Superior Court Judge Gladys Kessler for violating a no-strike injunction.