A family's tradition of sending its children to the exclusive Georgetown Preparatory School ended yesterday when a Montgomery County judge refused to reinstate a student expelled because of his involvement in a hazing practice that school officials denied knowing about.

Calling the hazing outrageous, Circuit Judge J. James McKenna denied an injunction request that would have forced the school to readmit the 17-year-old junior until a full hearing could be held.

"The public interest would be ill-served if I were to put this young man back into school tomorrow," McKenna said after a six-hour hearing. "This is a classic example of folks losing their moorings."

The student was one of four dismissed in December after allegations that they had engaged in a hazing practice called "butting," in which a student is held down while another student places his naked buttocks close to the victim's face.

The student was visibly upset by McKenna's ruling. "We're just the guinea pigs," he said, adding that many of the school's 400 students had observed or participated in butting.

The lawsuit, filed by the dismissed student and his parents, said butting was a well-known form of physical harassment at the school.

But several school officials testified they had no knowledge of the practice until last month. The school's president, the Rev. Thomas E. Roach, called the hazing "a serious insensitivity to another human being."

The dismissed student's father, a history professor at the University of Georgia and a 1948 graduate of Georgetown Prep, said he was disappointed with his alma mater.

"I don't think too kindly of the school," he said. "I plan to have my name expunged from the rolls." The man testified yesterday that 19 family members had attended the private boys' school on Rockville Pike.

The hazing allegedly took place Dec. 16. A 15-year-old sophomore who passed out after drinking too much later accused four other students of butting him on several occasions. The student who filed the lawsuit was dismissed Dec. 20 after Roach denied an appeal.

Yesterday, the expelled student testified he had watched only one butting involving the 15-year-old student in November. He apologized and said if reinstated, he would try to prevent other such hazings.