Alexandria School Superintendent Robert W. Peebles said last night that his staff was moving fast to deal with the gap in test scores between black and white students in the city's school system, which is the widest in the Washington area.

"Principals and teachers have developed plans for improving minority achievement in every school," he said in a 15-minute presentation to the city School Board. "We are not alone in this effort. It is an urgent problem across the urban parts of this country. What improves instruction for minorities improves instruction for all children."

Among the measures that Peebles outlined in his speech:

*Saturday morning workshops for parents of children achieving below their grade level.

*Introduction of a computer program designed by IBM to help kindergarten and first-grade children develop language skills.

*Increasing efforts to stress the classroom achievements of minority students.

As he has done repeatedly in the past, Peebles emphasized that test scores will not improve overnight and that better communication with parents will be necessary. But he added that the staff has already made a strong effort to use test scores to identify the weaknesses in course offerings that might contribute to problems for minority students and to look more seriously at the problems of individual students.

"We've been working like dogs on this task," said James P. Akin, the school system's executive assistant for research. "Everyone is on the right track now. We are talking about the problem and trying to change directions. No more sweeping it under the rug."

At the beginning of the meeting, the School Board advisory committee on special education held a ceremony to celebrate the 10th anniversary of PL 94-142, the federal law that established the right of all handicapped children in the country to a free public education. In more than 1,300 of the public school systems 10,000 students now receive some form of assistance under the law.