Led by their 18-year-old school president, an estimated 700 to 750 students at J.E.B. Stuart High walked out of their classes yesterday in a dramatic display of support for the salary demands of protesting Fairfax County teachers.

Stuart principal Richard W. Johnson, calling the walkout "unfortunate and counterproductive," said all those who participated in the protest will be suspended for five days.

With hundreds of students gathered around him, and some of them debating whether to stay out or go back in, Terry Shea, president of the Student Government Association, told them:

"If you're afraid of being suspended, I don't care. You've got to make the decision yourself. This is for the teachers."

A wave of cheers broke from the crowd, standing in a grassy, amphi-theater-shaped clearing in Stuart Park across from the school. In front of the rally, a group of students rode by in a blue pickup truck. From the bed of the truck they hoisted a hand-painted sign that read: "If you can read this sign, support your local teacher."

The walkout underlined the turmoil in the county school system since the teachers were told their cost-of-living raise would be held to 5.15 percent.That is half the current 10-to-11 percent inflation rate in the metropolitan area.

Pay-related teacher protests also have been held in Arlingon and Loudoun counties and in Alexandria this spring.

Though the entire school was emptied by a false fire alarm - Shea said he did not know who set it off - almost half heeded Shea's exhortation to stay out for the rest of the day.

Preparations for the walkout, which dwarfed earlier protests at other schools earlier this month and in April began earlier in the morning when Shea adn other organizers began slipping fliers under classroom doors.

" . . . I urge you to show your support for the teachers' cause by walking out of school," Shea said in the signed flier. "Go to your lockers and get all needed materials for the week-end. We are not going back to school until Monday."

Johnson said he didn't know if any additional disciplinary action would be taken against Shea, a senior who plans to study political science at the University of Virginia starting next fall.

While the earlier protests seemed as much prompted by spring fever as sympathy for teachers, yesterday's had and unmistakable political content repeatedly emphasized by Shea.

At the rally in the park, he reminded the crowd: "The point of why you are here, I don't want you to forget, is not because it is a nice day and the sun is out. You are here because of what the school board did last night."

On Thursday night, the Fairfax school board, after listening to teachers testify they need more money to survive, stuck by a budget that would hold the teachers' pay increase to 5.15 percent.About 1,500 teachers jammed Lake Braddock High School's field house during the session.

After the earlier rash of protests at other schools, Shea said he discouraged any such action at Stuart. Instead he helped organize a petition drive in support of higher teacher salaries that was signed by 1,200 students. He and other students took the petition to one of two meetings they had with John F. Herrity, cahirman of the Fairfax Board of Supervisors, and School Superintendant S. John Davis.

But after what the school board did, I didn't think I had any alternative but to call for the walkout," Shea said in an interview after yesterday's rally.

Shea obviously had the support of many of the students. "That's really courageous what he did," one said. "He's cool, he's okay," another said.

Even inside the school, the Shea-organized walkout had broad support. "I think we should walk out," said freshman Eileen Kelly, one of the handful of students who stayed behind in teacher Irvin Solomon's geography class. "But I don't want to be suspended."

There was another side, though, in the classroom. One of the four opposing the walkout was Mary Sofet, who said. "I support the teachers, but I don't think we should walk out. We won't get anything accomplished."

Robert Hicks, president of the Fairfax Education Association, which represents 6,500 of the 7,000 Fairfax public school teachers and has voted to "work to the rules" in protest against the 5.15 percent pay increase, was not pleased by the walkout.

"We don't support it," he said. "As far as helping us, I don't think it will do that much. All this can do is crete anatagonism.

Continuing its own protest action, FEA leaders have decided to ask their members to vote no confidence in the school board and reaffirm their earlier no-confidence vote on Superintendant Davis, who is leaving July 31 to become state superintendent of schools.

Jacqueline S. Benson, assistant superintendent of schools, speaking for Davis, who was out of town, said: "I am disappointed that many students would go along [with the walkout.] "There are more appropriate ways to protest . . . using political process."

Benson, like principal Johnson, said all students who failed to return to school after the false alarm will be suspended for five days. The suspensions will require rescheduling of some of the seniors' exams, which were to begin next Wednesday.

Johnson said that the suspensions will be kept on file in his office for a year, but will not become part of the affected student's records or to be sent to any college.

One of Shea's organizers, Chris Nathan, said they decided on the action out of frustration. "It came to the point where there was nothing else to do." CAPTION: Picture 1, Students milled around the parking lot at J.E.B. Stuart High School yesterday, before more than 750 of them walked out in support of Fairfax teachers' pay raises. By Bill Ennis for The Washington Post