When a lovelorn 18-year-old stormed into Lake Braddock Secondary School three years ago and took 10 persons hostage with a deer hunting rifle, it was Principal John C. Alwood who calmed the distraught teen-ager through the long ordeal.

"The next day, everyone said: 'Typical John Alwood,' " said Delores Bohen, spokeswoman for the Fairfax County school system. "Only John could have done it."

Now, after 13 years as principal of Northern Virginia's largest school, Alwood says he is leaving the prestigious position to return to the classroom.

"I'm doing it because it's important, and not for any other reason," Alwood said yesterday.

The 58-year-old principal surprised his administrative staff Monday when he announced that he is leaving the principal's office at the 4,720-student Fairfax County school to teach mathematics at the county's new Thomas Jefferson High School for Science and Technology in Annandale. Yesterday he repeated the announcement to the school's 250 teachers, who gave him a standing ovation.

"Teaching is the most important thing we do," Alwood said. "Lake Braddock, because of its size, has limited my contact with students. Here are some kids I can really work with, be their mentor."

Alwood's decision to return to the classroom is unusual in a profession where the usual career route progresses from teacher to principal. Spokewoman Bohen called the move "incredible."

Alwood would not comment on his salary in the new job. But Warren Eisenhower, the school system's assistant superintendent for personnel, said Alwood will not take a cut from his current annual salary of about $60,000 -- several thousand dollars more than the top of the teacher scale -- because his job will be a one-year appointment to demonstrate the results of his continuing research in the field of algebra.

Alwood said he has a year or two to go until he is eligible for retirement from the county school system. "I'd like to make sure I leave it feeling good."

One of the appeals of Thomas Jefferson, he said, is that it is a competitive-admission school for top math and science students from throughout Northern Virginia. "I wouldn't have done it for anyplace," he said.

Alwood joined the county school system in 1966 as principal of Edison High School. His education career began in 1949 in Michigan, where he taught mathematics. He left teaching in 1962, but he said he has tried to spend some time in the classroom each year to keep his skills from rusting. His biggest adjustment, he said, will be "getting to realize that the schedule I follow is not mine to set."

When he announced his plans to his wife and four children at Christmas, "They recognized teaching is tough," Alwood said. " 'Can you hack it, dad?' I can hack it."

Because Alwood has been in charge at Lake Braddock since it opened in the Burke community 13 years ago, his mark has been imprinted solidly on the sprawling school.

"His personality held the school together, not any rules and regulations," said Ann M. Jaekle, principal of one of Lake Braddock's six subschools. "He communicated a respect for people and faith in people and an optimistic belief that good would prevail.