IT'S ALWAYS SAD that a high school senior, when June comes, should fail to graduate. But it would be a great deal sadder if the schools allowed mere attendance to entitle a student to a diploma. This year some 17 percent of the Washington high schools' seniors will not be allowed to graduate.

No doubt it is hard on them to be told that they will have to repeat courses. But it would be a great deal harder on them, and dishonest on the schools' part as well, to send them into adult life under false impressions about their performance. Pat Brown, an English teacher at Ballou High School, had it exactly right when she said to a reporter that "you don't want them"--the failing students--"thinking that they are equipped for the dreams they have."

Some kinds of failure can be remedied. A course can be repeated in the summer, and a test can be taken again in the fall. It's harder to remedy the damage done by a complaisant school that, to avoid quarrels with students and their parents, simply gives everyone a diploma and lets all go on their way.

The cases of failed students do not, incidentally, have much to do with the system of basic competence tests that have now been adopted throughout the Washington area. Passing scores on those tests are, so far, decidedly low in relation to most people's expectations of a high school graduate, and the tests' principal purpose is to call attention to deficiencies in time to do something about them. The number of students unable to pass them at the end of their high school years is very small. But the course requirements eliminate a good many.

That always raises, at this season of the year, questions of fairness. The test of fairness, in withholding a diploma, is whether the student had adequate warning he was in trouble; schools generally go to great lengths to provide plenty of notice of low grades. Perhaps one of the most important lessons schools have to teach is that there are certain consequences to the habit of ignoring warnings. It's a lesson best learned early when, as in the cases of high school kids who have failed tests, there is still time and the opportunity to try again.