THE DISTRICT schools' new promotion policy is headed into trouble, some of the principals and teachers complain. Pardon our perversity, but those are good signs -- both the trouble and the complaints. When they say some children won't be promoted this winter, it is in fact promising, not dispiriting, news. For if everyone were promoted, you'd know that the whole venture was a fraud. In fact, the grumbling from principals, and the exasperated warnings from teachers, can be taken as solid evidence that -- so far -- the process is working.

In any school, some children learn quickly and some very slowly. There's always a terrible temptation to pass everybody along from grade to grade, automatically. That avoids trouble -- all the tears and parents' protests and accusations that can be expected when a child has to repeat a year. And that's how some children eventually get high school diplomas -- in the same spirit of mindless good cheer, through the same policy of avoiding trouble -- when they can neither read competently nor do simple arithmetic. Is it realy kindness to set children adrift, illiterate, at the age of 18? Can anyone really think that avoids trouble?

This year the city's schools decided that children in the first three grades would be measured in reading and arithmetic every half-year. Those who do not meet the standards will repeat the semester. One principal in a fit of bad temper suggested that some will be in the third grade until they grow beards.

Teachers complain that the budget cuts have deleted the specialized teachers who were to work with the children falling behind. But other school systems, less well staffed than Washington's, have successfully met this kind of standard. The bearded third grader usually turns up in hypothetical form when the standards are first applied, but rarely appears in fact. Failure rates are often high at first, as they will be in Washington. Then everyone, including both children and parents, begins to understand that progress requires more effort. Failure rates usually drop rapidly.

Children have a right to be taught to read. In the absense of a child's crippling handicap, that's a right to be enforced, and the city's schools are now beginning to enforce it. There's trouble ahead, true enough -- but it's the right kind of trouble, and it's welcome.