A HIGH SCHOOL DIPLOMA ought to certify that the graduate has reached a certain acceptable level of competence in his education. Unfortunately, the present practice usually amounts to giving a diploma to anyone who sticks around long enough. That's unfair to employers and the community. And it's especially unfair to those youngsters who think they are getting an education - and discover too late, only after graduation, that they are wrong.

Competence tests, as a requirement for graduation, raise prickly questions for school boards. Children's failures are often the schools' failures as well. But throughout the country, school systems are edging toward the idea of graduation tests. Virginia's state board of education has become the leader in this region.

A year ago the Virginia board voted to let local school systems impose their own tests, beginning in 1979. But on further reflection, it has decided - correctly - that leaving the hard questions to the local boards is not necessarily helpful. One of the gravest defects in Virginia's educational policy over the years has been its toleration of vast disparities in quality between one local system and another. There's a need for a basic statewide standard, and it ought to be spelled out at the point at which students enter high school. That's why the state board has now pushed its target back to the class of 1984, the children who are in fifth grade today.If the thing is done properly, it will not be just one test given right before graduation, but a series of tests beginning early in high school to spot trouble in time to try to remedy it.

Maryland has decided against graduation tests, preferring instead to set a standard for each grade. But it's still not entirely clear what happens to the children who consistently can't meet those standards. The Washington schools pay their respects to the principle of competence tests, but don't seem to be doing much about it in practice.

A lawsuit in a Long Island, N.Y., school district illustrates the point. A young man graduated from high school there with a fourth-grade reading ability, and he's now charging the school with educational malpractice. A student can't claim a legal right to know how to read. Some children have handicaps that will always make reading difficult for them, just as some will never learn to sing on key or balance a checkbook. But children and their parents are certainly entitled to warnings when trouble appears. When a school system amiably gives an illiterate child normal grades and passes him along routinely, it is evading a basic responsibility. Competence tests make evasion harder. Not incidentally, they also help to guarantee that the successful students' diplomas will be worth having.