Sandy N. McDonald, the Wheaton assistant principal charged with firing a gun six times into a crowd of students last fall and wounding one of them, pleaded guilty to one count of assault in Montgomery County Circuit Court yesterday.

Circuit Court Judge David L. Cahoon said he would wait until a presentence investigation was completed before accepting the plea and set sentencing for Feb. 20.

McDonald, 53, has been on administrative leave with pay since Oct. 26 when he raced out of his home near Wheaton High School, and fired six shots at a crowd of students, some of whom he claimed had muttered racial slurs at his daughters as they walked home from a pep rally.

McDonald, who had a reputation as a disciplinarian at Wheaton, had reported more than a dozen incidents of racial harassment and vandalism to Montgomery County police prior to the incident.

McDonald had originally been charged with assault with intent to murder, but the charge was lowered to assault under an agreement worked out with the Montgomery County state's attorney's office. At yesterday morning's hearing, McDonald read a statement explaining some of the factors that led up to the incident, according to Charles Chester, a law associate of McDonald's lawyer, Maryland state Sen. Victor L. Crawford.

The assistant principal is free on $5,000 bond. School officials will not remove him from administrative leave until the case is finally decided in the courts, a spokesman said.