Why does one predominantly black school in a low-income neighborhood turn out youngsters who score above the national norms while its neighbors, serving similar populations, do poorly?

It's no mystery, says one group of experts: it's the principal of the thing.

Barbara A. Sizemore, former superintendent of the D.C. public schools and now with the University of Pittsburgh, headed an investigation of three high-achieving Pittsburgh schools. What she found has become almost a clich,e: the principal makes the difference.

Specifically, the study, funded by a grant from the National Institute of Education, found that principals of high-achieving black schools tend to be authoritarians who believe, and are able to convince their staffs, that poor black children can learn; who dare to risk differing with (and even defying) the central administration for educational ends, and who manage to find ways, often in defiance of the rules, of getting rid of unsatisfactory teachers.

The conclusions echo those of a number of professional and lay investigators who have studied what Sizemore calls the "abashing anomaly" of successful inner-city schools. But the present study takes a deeper look than most at the particular styles and techniques of successful principals.

These principals, the study found, tend to assume responsibility for student discipline, attendance and resolution of parental conflict, leaving teachers free to devote most of their time to teaching and instructional preparation. They provide frequent and prompt evaluations of teachers, offering expert help where it is needed, but also "persuading" weak teachers to transfer. They manage to involve parents "in some participatory and meaningful way" in the school program, whether as partners or as "clients who were expected to give the school support in exchange for the education of their children." They place heavy emphasis on phonics, and on celebrating the black experience.

While all three of the successful schools used ability grouping (and self-contained classrooms), two of the three rotated the assignment of teachers to the high-ability classes. In the third, and most successful, school, the assignments were permanent, thus bypassing the "yearly struggles to avoid the 'ding-a-lings,' as one teacher labeled the unwanted students."

Of particular interest for Washingtonians who recall the protracted administration-teachers' union fight over an extended work day, teachers at all three of the high-ranked schools routinely devoted extra, uncompensated time for special subjects, small-group instruction and tutoring.

This was possible, according to the Sizemore report, because all three principals succeeded in enlisting their staffs in what might be called a conspiracy of achievement. The teachers gave the principals their loyalty and support in exchange for the principals' assumption of discipline enforcement and protection of teachers who dared to defy the central administration by, say, using unauthorized teaching materials. ("One 20-year veteran acknowledged having served on several reading textbook committees where teachers were urged by central office personnel to choose texts other than those the teachers wanted because of certain benefits offered the school district by the publisher.")

Why does Sizemore call these successful schools "an abashing anomaly"? Because her observations "seem to point toward a tendency to hide the high-achieving black schools; to ignore their contributions to teaching and learning, and to pretend that they simply do not exist. To admit their presence is too embarrassing, an open admission that the decision to improve the quality of instruction for black students is a political decision and not an educational one."