PHILADELPHIA -- A defiant teen-ager, suspended from school for spitting profanity at a teacher, is back in the principal's office. He sits at his mother's side, eyes on the floor, asking sheepishly if he can return to school before graduation.

For Davis Martin, principal of one of the city's biggest, poorest, roughest high schools, it is a familiar scene. "Come on," he says. "We got right down to senior year and graduation. I told you, you can't let anything come between you and graduation . . . . I'm looking for you to be my success story."

This boy has been in and out of the school for three years, in trouble for fighting, in trouble for harassing a girl, in trouble for what Martin calls "trifleness." Martin knows the teachers and vice principals won't like it, but he decides to give the boy another chance. How, he asks, can a child learn if he's out on the streets?

Much has changed for Davis Martin in 15 years at University City High School. When he arrived, students carried weapons in their book bags and black gangs fought in the corridors; tension was building between the blacks and the Asians.

Leadership then meant keeping the chaos from erupting. But like many big-city high schools, University City has been tempered and calmed. Because of the changes, Martin must change too. Leadership now means more than keeping the peace: it means keeping the students in school. The school doesn't need a general for its principal. The young people are surviving; now they need to learn.

Principals have always symbolized the school, set the tone, made the rules, kept the bathrooms safe. But the job is changing, especially for those who run big-city high schools.

Most urban high school principals were hired because they were hot-shot managers or strong disciplinarians, not because of their reputations as educators, said Santee C. Ruffin Jr., an official with the National Association of Secondary School Principals. As a result, he said, "We have a number of quiet and safe schools mired in low expectations and with little being done to strengthen the teaching and learning experiences."

Principals like Martin are being told to abandon methods that have served them well for years, worry less about rules and more about math, spend less time in detention halls and more in the classrooms.

The new expectations for these administrators are springing not only from a tentative renewal of urban schools, but from drastically changing notions about the role and importance of principals in the learning process.

At a time of substantial turnover -- nearly half of the country's 100,000 principals will reach retirement age in the next seven years -- the new concept of principals as educators rather than managers is creating a remarkable period of transition for the office.

It's an altogether different mandate, and at age 56, with three decades in the school system, Davis Martin is feeling his way.

"We've gotten over the violence and the gangs. Now we're getting to what we should be about," he said. "I feel comfortable taking on the task I see before me."

But a consensus in scholarly circles that principals should change their style doesn't mean that the people in those offices will adjust.

"It's a hard transition for people to make," said Kenneth Tewel, a professor at Queens College of the City University of New York. "They need to catch up. They need to read a lot, be retrained.

"There's a natural reluctance," he added. "We want to keep doing what we do well."

Also, the new formula for academic success may come easier to principals at other schools, where the legacies of poverty are less intrusive than they are at University City.

Consider the changing, but still difficult, climate that faces Martin. His school draws students from a section of Philadelphia that ranks high on all the indices of misery: infant mortality, police calls, teen-age pregnancy. Of its 1,600 students, half live in families that are on welfare; 83 percent are black, 16 percent Asian and a handful white. Fewer than half the ninth- and 10th-graders are passing their basic subjects and the school dropout rate is twice the city average.

There are still bars on the classroom doors.

But, like many other big-city schools, this one is calm compared to the 1960s and 1970s, when raging urban violence spread from the streets to the schools, and settled there.

A 1984 report by the Ford Foundation discovered "a national phenomenon of renewal" in the nation's high schools. And in urban high schools, long perceived as crippled and failing, the study found "that a goodly number . . . have come some distance from the recent past."

For Martin, the recent past was punctuated by bitter racial animosity. Faced with almost daily fights between black students and the growing number of Asians assigned to University City because of its English-language program, Martin called an assembly. He set up two microphones, then surveyed the students about their feelings. He described the session:

"What is it you don't like about blacks?" he asked the Asians.

"They're taking money from us. They're loud. They interfere with our studies."

"What problems are there with the Asians?" Martin asked the black students occupying the other half of the auditorium.

"They're getting over on us. They understand more than you think. If I'm out in the hall, you suspend me. If he's out in the hall, you show him the way to class."

Teacher Edie Saltzberg said there was a "palpable difference" after this period of soul-baring. "There hasn't been an incident for years. They just pass each other in the halls."

Today, Martin is still diplomatic and accessible. His office is open, and a teacher or a parent or a student is always peeking around the doorjamb. They want to talk about everything from bus tokens to field trips to substitute teachers. No detail is too small.

"They don't have to go through a secretary to see me," he said. "They can see me faster than a vice principal, faster than a teacher . . . . I tell my fighters, 'Before you fight, come see me.' But if the door's closed, how can they see me?"

The way Martin sees it, when he is taking care of these details he is keeping his staff happy; students know he cares and parents feel they have a friend at the school.

"I haven't been one to give the store away," he said. "I don't like to ask what's going on in my own school."

Martin is the "classic example" of the principal in the midst of changing expectations, according to Tewel, the Queens College professor. "He inherited a school in awful shape. He had to get the place in shape . . . and he did."

Tewel is a "mentor" to Martin in a training program designed to help principals make the transition from management to instructional priorities. He said Martin was sent to the program, which is sponsored by the National Association of Secondary School Principals, because "the superintendent feels he needs that extra push and support . . . to hone his instructional support and do something about those horrible scores . . . . That's the new agenda."

The new agenda is drawn from a flood of recent research on academic achievement, much of which points to the principal as the key player in the learning process.

The "effective schools" research of the late 1970s cited strong leadership by principals, especially in instruction, as a critical component in improving performance. According to these theories, it is up to the principal to create an orderly climate, emphasize basic skills, set high expectations and carefully monitor student performance.

In the past, principals were expected to use whatever curriculum was sent down from the district office. According to the new theories, they should be given more discretion over curriculum, be more familiar with current research on teaching methods and more willing to involve teachers in designing and planning programs.

Principals traditionally have operated on the assumption that leadership meant high visibility, standing in hallways and moving through the building making momentary contact with as many students as possible.

The "new" principal, according to Tewel, meets with fewer students and teachers, but interacts with them in greater depth on educational, not organizational, matters. This means taking a minute to look through a student's notebook, or spending more time observing teachers at work in the classroom.

This increasing focus on principals as educators comes after years of assumptions that superintendents, district instructional experts or teachers were most important in determining whether students excelled in school.

Now, research and observation has shown that the individual school is the important "unit of analysis" in learning, with "the principal as head of that unit and responsible for its success or failure," said Chester E. Finn Jr., assistant secretary at the Department of Education. "You practically never find an effective school without a very good principal."

Martin is well aware of the new expectations. He is adopting a writing program that eventually will require students to complete writing assignments in every course, from physical education to the physical sciences.

He has adopted a "school improvement" plan that sets specific learning goals. For example, ninth graders should be able to write an eight-sentence paragraph with no sentence fragments. Foreign students are expected to master 80 percent of their English vocabulary lists.

Martin is working on the new formulas for academic success, but he has also focused on other projects, opening a health center to deliver basic health services and proposing an in-school day-care center. This will allow the students he loses to teen-age pregnancy to return to school, have a place to keep their babies and learn the skills of being a parent.

The day-care proposal, which is waiting for outside funding, is Martin's baby. If he wins on that, he'll be making what he believes is the most important instructional contribution to these students. He'll get them back into the classroom and head off some of the tragedies -- child abuse, drugs, poverty -- that await many teen-age mothers.

"We have to think in terms of bringing kids in rather than driving them out," he said. "I've got to compete with the corner. I'm competing with drugs. I'm competing with teen-age pregnancy."

These programs come together slowly. So does a new leadership style, after half a career in the same office.

"I'm not ready for the Super Bowl," Martin said, but he added: "We're moving into a good academic environment . . . . We're moving in the right direction."