The Prince George's County public school desegregation trial is over, and the most pressing question never was debated.

U.S. District Judge Frank A. Kaufman, who issued the original desegregation order in 1973, is expected to rule shortly on the Prince George's NAACP's complaint that the county schools were never fully desegregated. That was the question before him.

But the question that surely must have been in the minds of the plaintiffs--how best to educate the black children of the county--had no place in the eight-week trial. That, after all, is not a constitutional issue; desegregation is. As a result, lawyers and witnesses for the NAACP and the county spent two months arguing not about education, but about statistics.

The key statistic is this: the 163,000-pupil school system that was 22 percent black a decade ago when the case was first decided is today a 116,000- pupil system that is 52 percent black. White families have been leaving Prince George's, birth rates have been dropping, and blacks have been moving into the county, mostly from the District.

As a result, there was no way the county could have maintained the orignally mandated racial ratios --even assuming Kaufman's order had been carried out with absolute scrupulousness. Should county officials have changed busing patterns to keep up with the racial changes? The NAACP obviously thinks so, and Judge Kaufman himself thought that that had been a part of the original understanding, though it apparently was not part of his order.

Indeed, it is hard to see how it could have been. His original finding of unconstitutional segregation was based on evidence that official actions had produced schools that were disproportionately white or black. If today's racial isolation is the result of changing residential patterns, there is no obvious constitutional requirement that the county do anything about it.

Kaufman himself made the point with regard to another set of statistics: numbers showing that black students were disproportionately assigned to programs for the mentally handicapped while only 12 percent of those in the talented and gifted program were black. Those numbers, the judge said, do not "even come close" to proving a constitutional violation.

The crucial point, however, is that racial statistics and busing patterns offer no useful clue as to the quality of education Prince George's affords its black students. Indeed, careful observers of the two-month trial could deduce from the testimony virtually no clue as to how well the county is educating its children, black or white.

It may be that such questions cannot be addressed in court. But it does seem clear that they ought to be addressed somewhere. Wouldn't it have made more sense for the NAACP to apply its dwindling financial resources to finding ways to improve the education of black children, rather than merely integrating them? Wouldn't it have been useful to look at such factors as teacher qualifications, school facilities and resources and academic improvement, with a view to providing a better education for black children? If, as is generally accepted, parental participation is a critical factor in student achievement, shouldn't someone have asked whether busing children to distant schools reduces the likelihood of parental involvement?

My guess is that black parents in Prince George's want what parents everywhere want: the best possible education. It's hard to see how that desire is served by the NAACP's assumption that good education can happen only in classrooms with the "right" number of white children in them.