The D.C. Office of Human Rights has ruled that a woman school administrator was a victim of sex discrimination when a man was promoted over her in 1978 to an assistant superintendency of the city school system.

The Human Rights office also ordered that Marilyn T. Brown be given the higher rank and an estimated $7,500 in back pay.

The ruling, signed Monday by Human Rights office director Anita Belamy Shelton, held that former D.C. School Superintendent Vincent E. Reed acted wrongly in choosing William J. Saunders over Brown to fill a vacancy as assistant superintentent in charge of the school system's Region 5.

Brown was the first choice of a selection panel composed of parents, administrators and the then-president of the school board, Conrad Smith. Saunders was their second choice. Under school rules, Reed had the option of choosing any of the top three candidates.

Reed, who retired at the end of 1980, voiced disappointment at the decision. He said the biggest difference in qualifications of Brown and Saunders was the she had a doctorate and he did not. Both are long-time employes of the system.

"My record as superintendent (in the appointment of) women has got to be the best in the country," Reed said last night when told of the decision. He said he had appointed women as vice superintendent and as heads of offices in charge of the budget, personnel, research and staff development.

Acting Superintendent James L. Guines, who succeeded Reed and must decide whether to accept the order or to appeal it, declined comment. George Margolies, the school board lawyer who represented Reed in the case, said he would study the decision and recommend to Guines whether he should appeal the case to the courts.

Brown currently is assistant director for instructural management in the school system's instructural services center. Saunders, after serving several months as assistant secretary for Region 5, which encompasses much of north central Washington east of Rock Creek Park, became Reed's executive assistant. He retired at the same time as Reed.

The petition to the Human Rights office was brought in Brown's behalf by two volunteer attorneys for the American Civil Liberties Union, Marie N. Doland and Judith Bailey. Announcement of the decision was made last night by the ACLU, which estimated the amount of back pay due Brown.