A Prince William County elementary school principal discriminated in disciplining a black third-grade student last fall, the U.S. Department of Education's Office of Civil Rights has found.

On one occasion, Principal Richard Keeler disciplined the boy without fully investigating the facts, and in a second instance he treated the child more severely than a white student who committed the same offense, said Robert A. Smallwood, the department's civil rights director for the region.

Smallwood's findings were contained in a letter to county Superintendent Edward L. Kelly. As a result, school officials have agreed to expunge the black child's disciplinary file of any mention of either incident. Kelly also agreed to have a school official review any other disciplinary actions Keeler may have taken against the youth this school year, and to continue the review next year.

Keeler has been reassigned to another elementary school next year, an action Prince William officials said was not tied to the disciplinary flap.

The investigation was prompted by a complaint from the boy's mother to the Civil Rights Office, which receives more than 700 such complaints each year.

Keeler, who was chosen by his colleagues to receive a distinguished educational leadership award from The Washington Post in 1988, could not be reached for comment yesterday. A former school superintendent in Warren County, he was principal at Westgate Elementary for three years before his reassignment to Mullins Elementary for the coming school year.

Mary Wilson, the mother of the boy disciplined by Keeler, said yesterday that she is satisfied with the federal findings but was "disturbed that Dr. Keeler would be moved to a new school . . . . If he has discriminated against a child, he should not be working with children."

Kelly, however, called Keeler an exemplary principal who has "earned the opportunity to open a new school" this fall.

Sidney Yeldell, human relations supervisor for Prince William schools, said the school system is contesting some of the findings, but he declined to be specific. Yeldell said school officials will meet with Smallwood soon to discuss the matter.

According to Smallwood's letter, Wilson's son, who is now 9, was accused by a white parent of chasing her daughter home from school one day last September. Keeler issued a warning to the boy that was put in his school disciplinary record.

In interviews with the Office of Civil Rights, the boy denied chasing the girl or telling the principal he had. A witness, whom Keeler had not interviewed, told the investigator that both children were running and that no one was being chased.

In the second incident, the parents of another white student told Keeler that Wilson's son had chased their son home from school with a stick. The federal investigator conferred with the two boys, and they admitted they both had sticks and had been taunting one another. The incident was recorded on the black child's discipline sheet but not on the white child's, according to Smallwood's letter.

"From our point of view, we don't have any problems with the whole process of the investigation," Kelly said yesterday. "We believe the situation was handled properly, although our conclusion was different."