For the crowd at Thomas Jefferson Intermediate School in Arlington yesterday, it was the first day of school all over again: the packed auditorium, the glad greetings after a three-month absence, the pep talk from school leaders.

But the 900 people who filled every chair, clogged the aisles and crouched on the carpet were teachers, not Arlington students, and the words from new Superintendent Arthur W. Gosling were aimed at the heart of their profession.

"What happens in a classroom between teachers and students is the single mission of a school district," he said. "What happens between those four walls is what we ought to be focusing our attention on."

Gosling was named last spring to succeed Charles E. Nunley, who left the superintendency June 30, at the end of his first four-year contract.

In yesterday's speech, Gosling's first to the entire Arlington faculty, he did not discuss specific educational goals but stressed teachers' roles as models, people whose optimism urges students to achieve.

"Students respond to how we do things more than to what we say," Gosling said. A teacher becomes "a human model that indicates commitment, energy, belief and caring for each of those students every day. Kids know if we care," he said.

"School staff have to believe in the future," he added. "You have to be optimists. That makes the difference between a school that contributes to our students' future and one that's just marking time."

The teachers, many clutching first-day-of-school schedules of meetings, responded with chuckles to a video-taped introduction of Gosling prepared by school staff.

The tape, with its own music soundtrack, showed Gosling in scenes from 1959, wearing heavy black spectacles, in his early teaching jobs in Ohio and as principal of a high school in Highland Park, Ill., assistant superintendent in South Orange and Maplewood, N.J., and, most recently, area superintendent in Fairfax.

In comments that alluded to tensions between teachers and Nunley that have arisen over the last four years, Arlington Education Association President Bonnie Pfoutz asked all the teachers to stand for a formal introduction to the new superintendent.

"Dr. Gosling," she said, waving a hand toward the packed room, "We teach the children."

She urged Gosling to "use us as a resource and involve us in all levels of decision-making," noting that "there has been a sharp distinction in the past between we who teach the children and those who make the decisions."

County Board Chairman John G. Milliken praised the school system as an important lure for the young families he hopes will move to Arlington in greater numbers.

"We are in the midst of a serious effort to attract more families to Arlington County. There is no better way to do that than to insure the continued excellence of Arlington schools," he said.

Gosling urged the teachers to strive for that excellence. "I ask each of you, as I ask of myself, to do [your] very best," he told them. "Go forth, do good, make a difference."