ABOUT FIVE years ago, the Washington area's suburban school districts began to focus on the disturbing achievement gap between average white and minority students. In standardized tests, the gap was more than 30 percentage points in Fairfax and Arlington counties, 20 points in Prince George's and 16 in Montgomery County. By far the worst gap, however, was found in Alexandria -- 48 points. Why?

Unlike other suburbs, Alexandria's black population is generally very poor with only a small middle class. Among the suburbs, Alexandria has the highest percentage of poor students and many others with limited English skills.

Alexandria's response four years ago was to hire Paul W. Masem as superintendent; his specific charge was to improve minority student achievement. The embattled Mr. Masem has now been accused of neglecting the needs of other students, particularly whites. His plight highlights the folly of appointments in which a superintendent is brought in to tackle one issue: He now stands accused of trying too hard to solve what he was hired to solve. It also appears to be a bum rap.

In 1986 white students in the Alexandria schools scored at the 73rd percentile on standardized tests. By last year, they had improved by 10 points. Alexandria's white students did better than whites in the highly touted Fairfax County schools at every level on the last round of tests. The city has one of the highest percentages of students whose goal is to attend college, and scores of Alexandria students have gained admission to the nation's most selective universities.

If Mr. Masem deserves criticism, it would involve the fact that the gap between black and white students is still huge, although smaller. But black students have shown improvement, moving from the atrocious level of the 27th percentile among 11th graders in 1985 to the 36th.

When one considers the area's superintendent posts, it may be that Mr. Masem's task is second in difficulty only to that of his peer in the District. One of the greatest challenges to a superintendent like Mr. Masem, as these jobs become more highly politicized, is to persuade parents that it is not necessary to deprive one child in order to improve the performance of another. In Alexandria, both white and black children have shown progress. For that, Mr. Masem deserves more credit and less criticism.