The Washington Teachers' Union is behaving badly. James M. Harkless and his fact-finding panel have provided notably fair and sensible recommendations that offer a firm basis for a settlement. The city's School Board has accepted it. But the Teachers' Union is responding coyly, with a vague suggestion of further negotiations in which the Harkless report "will be refined."

It's a risky game. The city schools are supposed to open on Sept. 10, and if there's no settlement the school year will begin with a strike. A strike in September would undoubtedly be very much like the one last March. It would change little to the benefit of either side, and the costs would again be borne mainly by the children in the disruption of their education.

Mr. Harkless has proposed, essentially, that everyone go back to the conditions existing before this long, unhappy dispute began. The School Board started it, you would have to say, with the best of motives and the worst of tactics. In its efforts to raise achievement levels, the board unilaterally canceled the teachers' contract and tried to impose a longer day and a longer school year on its schools. Since the board does not at present set teachers' salaries, there was no raise in pay to accompany this substantial increase in hours. The union's reaction was, naturally, explosive. As usually happens, a great variety of further grievances were drawn into the quarrel as the strike went on -- and on.

But the board will, at last, get a degree of control over the teachers' pay next year, when the city's new personnel law takes effect. It seems obvious that all issues of working conditions are best postponed until then, when the board will be able to offer something in return for the longer hours that it still wants. That's another reason to believe that a strike next month would be a futile as the one last spring.

Perhaps the union isn't serious. Perhaps it's just playing around with hints of threats. The language of its statement yesterday invites the inference that it wants the revocation of the board's program for evaluating teachers. Perhaps the union is only seeing whether the board can be pressed to back down on evaluation without a strike. But, as the union has good and recent reason to know, a threat sometimes takes on a life of its own, and strikes are easier to start than to stop.

The union says that it is "not interested in taking an action which would delay the opening of school." If that is the case, it can serve its interest best by joining the board in a prompt and forthright agreement ot take Mr. Harkless' advice.