Last Friday, as homecoming week at Wheaton High School came to an end, a diminutive assistant principal named Norris (Sandy) McDonald put aside his customary three-piece suits and decked himself out in jeans and ten-gallon hat in the out-of-character fashion of a cowboy.

In his eight years at the school, McDonald had carved out a formidable reputation as the school's disciplinarian, a role unpopular enough to get him hung in effigy in 1977. But that did not seem to trouble him on the last day of "spirit week."

"We joked," recalled principle Anson Wilcox. "He was in a nice mood. He told me, 'Andy, the kids have been more friendly today than in eight years."'

Nine hours later, as students milled in the streets near the school after a homecoming rally, the goodwill of that afternoon would seem like a cruel hoax. The October day that had begun so harmoniously ended with Montgomery County police arresting the black, 53-year-old school administrator on charges that he fired six shots into a group of white youths outside his home. McDonald said he was firing warning shots at men he saw in his backyard, but according to police, the group that he fired at included more than a dozen students from Wheaton High School.

For residents of Wheaton community, sensitive to residing in a lower- to middle-class community in an affluent county, the shooting was a troubling reminder that they are not immune to the racial tensions of the city. Even more upsetting was the principal's contention that the shooting was the culmination of eight years of racial harassment at his redbrick home a block from the school.

McDonald had meticulously documented a litany of harassment since 1972 and had reported 12 incidents in a period of five years to Montgomery County police. Many of his acquaintance had no idea of the incidents, but inside McDonald a fuse had been invisibly burning for years.

"After this many years the wheel just ran off," the thin, mustachioed assistant principal said yesterday, sitting in his living room with his wife, his son and one of his four daughters. "I began to feel like I was not providing protection for my family. Nobody really cares about us at all."

The cathartic outburst astonished students, teachers, and the quiet tree-lined communities that surround Wheaton High because most people say the school, on the surface at least, doesn't have racial problems, and because McDonald was respected as a professional educator. "I nearly went through my chair," said Wilcox, who visited McDonald Sunday afternoon after returning from out of town. "I've seen fights, but in 31 years of education, never a gun going off."

School officials, police, and the Wheaton Parent Teachers Association insist the incident was an isolated one, and is not indicative of serious racial problems in either the school or the community.

PTA officials are especially worried that the situation could endanger the vocational training center planned for the school where enrollment has declined from 2,400 in the early 1970s to 1,230 this year.

Although most people at Wheaton High say the racial aspect of the shooting has been exaggerated, a number of white students interviewed at the school yesterday charged that McDonald bent over backward to favor blacks -- slightly less than 20 percent of the school population -- and came down hard on white students with discipline problems.

While growing up in North Carolina and later while working in the South, McDonald was never harassed or made the object of racial slurs, he said yesterday. The trouble began in Montgomery County.

Shortly after he arrived at Wheaton High School in 1972, McDonald was beaten by three men who he does not believe were students in the school hall. They told him they were members of the Ku Klux Klan. "They said I was the first nigger vice principal in the school and they wanted to give me a little welcome," McDonald said yesterday.

During the last eight years, his car was painted with the letters "KKK", he was repeatedly subjected to racial epithets by students, the windows of his Cadillac and white van were riddled with bullets outside his house and he received a letter at school from the "KKK of Wheaton High School" saying he should be hanged. Last month, a stack of newspapers was placed against the house and set on fire and on the same night, two of the tires on his Cadillac were slashed.

Police spokesman Nancy Moses said McDonald reported 12 or 13 incidents to police during the last five years. But police have not arrested any of the vandals, partly because no one witnessed the incidents and partly because police in the Wheaton district get huge numbers of vandalism reports -- 2,300 in 1979.

The incident Friday night began when McDonald's daughters, Odette and Karen, both students at Wheaton High, were returning home from a pep rally and walked past a crowd of students one block from the school and around the corner from the McDonalds' home on Farthing Drive.

According to students who were there, one student who was standing by a car, drinking -- no one is saying who -- shouted a racial epithet and and called out, "There goes the McDonald clan. The white clan is better than the McDonald clan."

The two girls ran to their home and told their mother, Bertha, and she called McDonald, who was at a neighbor's house about eight blocks away.

On his way home, McDonald drove past the the group of youths, who were drinking beer.

"We started yelling at him because he was driving crazy," said Tom Carney, a senior at Wheaton High who was standing with about five other youths.

McDonald then parked his car in from of his small one-story brick home, three houses away from the students, went inside and emerged with a gun.

Students say they next saw McDonald running toward them. "I saw sparks coming out of the barrel," said Robbie Cross, a student who was standing on the corner. "He was saying, 'I'm going to get you, I'm not going to take it no more."'

The youths ran away from him, to the other end of the street, where they ducked behind cars and houses. "All these people came running, saying, somebody's shooting at us," said Pat Cottrell, a Wheaton junior.

Matthew Jones, who was standing near Cottrell, began yelling that he had been shot. 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.

Several students said McDonald, who fired about six shots, returned to his front yard where police arrested him. Jones, the wounded youth, said he and other students went to McDonald's house.Jones said he told McDonald, "Look what you did. You shot me." Jones, Cross and another student, Tom Carney, said they heard McDonald reply, "Next time I won't miss" and his daughter add, "Next time I see you I should shoot you between the eyes."

Yesterday, at his home, McDonald denied making such a staement to Jones and described a different version of the incident. He said the shooting had nothing to do with the students, and denied that the shooting was inspired by the racial slurs aimed at his daughters.

He said he actually was firing into the air to chase away six or seven men who had been in his backyard and who fired two guns at him.

"I've been going through that all the time," he said, referring to the racial slurs. "I didn't like it but it did not make me angry. I'm not angry with the students. This is an issue between me and the adults," who he says had fired shots at him.

Moses, the police spokesman, said police had found no evidence that anyone had fired shots at McDonald, or that anyone had been in his yard.

Several students who witnessed the incident said they did not see the men McDonald said he was chasing, nor any men with guns.

Yesterday McDonald said he was trying to become resigned to being harassed. "Nothing can be done," he said. "I can't stop it. We'll have to accept all the harassment that is placed on us."

Pending th outcome of an investigation by school school officials, McDonald has been placed on administrative leave. He will appear before a grand jury Thursday on charges of assault with the intent to murder.