An assistant principal at Wheaton High School has been arrested by Montgomery County police who said he raced out of his home near the school Friday night, yelling, "I'm tired of this" and fired six shots at a large crowd of students milling at the end of the street.

Sandy N. McDonald, 53, who police and faculty members said has been harassed by vandals for more than five years, was charged with assault with intent to murder by police who said one of the shots McDonald fired from a 22-caliber pistol grazed a 17-year-old Wheaton High student in the arm.

McDonald, who lives a block from the school and has reported a dozen incidents of vandalism and harassment, has been an assistant principal at Wheaton since 1972. A school official said he probably would be suspended with pay until the school system investigates the incident.

Although county police said they had not established a motive for the shooting, a faculty member said McDonald has told friends the incident was sparked when several students refused to leave his fenced back yard. Neighbors and students who witnessed at least part of the disturbance also said some young persons muttered racial epithets at two of McDonald's daughters as they walked home after a homecoming pep rally at Wheaton about 9 p.m.

McDonald, released Friday night on $5,000 bail, declined to discuss what happened. "We don't want to make a big deal of it," said a son, one of McDonald's four children. "My father's done lots, enough positive things for this school to fill a whole page."

Police and faculty members said some of the harassment aimed at McDonald, who is black, has been racial. In addition to the remarks students made to his daughters Friday night, police said racially derogatory notes have been left in his front yard.

A crowd of more than 50 students spilled out of the school parking lot after the pep rally, some looking for a party at a nearby house and others drinking beer and milling around cars parked along the street.

Shortly after 9 p.m., McDonald, whose home has been shot at and whose Cadillac has been the target of quick-dry cement, dashed out his front door and ran past three houses to the corner of Farthing and Fredale streets, where he encountered the first group of students.

"I was standing in my driveway and saw this guy running down to the corner waving a gun," said one neighbor."He was yelling, 'I'm tired of this . . .' and I could hear a woman's voice calling him to come back."

At the corner of Farthing and Fredale a knot of four to six students was drinking Colt 45 and Miller Lite in front of a house with a chain fence around the yard.

As the students saw McDonald approaching them, they ran down Fredale, toward a much larger crowd at the corner. It was then that neighbors said McDonald leaned up against the chain link fence and fired.

"I could hear shell cases hitting the ground," said one neighbor. "At first I didn't believe it, I thought it was the firecrackers they use to scare off starlings."

At the end of Fredale, 17-year-old Matthew W. Jones of 4905 Stickley Road, Rockville, stood next to his Volkswagen. "There were four of five guys up the street he was shooting at," Jones said.

"They ran down toward us, and then he started shooting at the biggest group. You could hear bullets hitting the trees and people ducked behind cars."

Jones said he felt a sharp stab, and saw a hole in his jacket, made by one of the slugs. "It just hit me in the arm," he said. "It felt like somebody hit me with a stick. It made a pfftt sound. I didn't know what to do. Nobody believed I was hit."

Jones was taken to Holy Cross Hospital, treated for a superficial wound, and released.

School spokesman Kenneth Muir said, "There are two things we don't know: what happened with the daughters and to what extent his behavior was motivated by past harassment." As Wheaton's assistant principal, McDonald was involved in discipline at the large high school, a position which typically earns few friends with students.

He had formerly taught government, psychology and history and worked as a principal in schools in North and South Carolina, and had formerly been employed by the Colorado Dept. of Education.

Wheaton biology teacher J. Dewey Brown said yesterday that McDonald "is a very professional person, highly respected by our faculty. I guess any parent is going to respond if there is harassment of his children."

Physics teacher Don Barron, who joined the Wheaton staff the same year McDonald did, said last night that McDonald had been physically assaulted by rebellious students and that newspapers were spread out on his lawn recently and set afire.

"It's basically Sandy's character in school," Barron said, describing him as a slightly built well-dressed administrator who is both admired and feared. "He's very strict, but fair. Those who wish to defy authority find him easy to pick on. He does his job an takes the abuse for it."