About 2,000 Fairfax County teachers rallied yesterday to threaten a work-to-the-rule job action unless the board of supervisors gives them more than the 5.15 percent raises recommended by the school board. The teachers want 9.4 percent raises.

They voted to authorize the leadership of the 6,500-member Fairfax Education Association to organize the "strongest possible action" April 24, the day after the supervisors are expected to take their final vote on the fiscal 1980 budget.

The "strongest possible action" is understood to mean a work-to-the-rule protest in which teachers would refuse to volunteer for unpaid activities such as being club advisers or class sponsors. Teachers would do their planning and grading during school hours instead of at home, slowing classroom procedures. Some sports might be hampered by the refusal of coaches to work with teams.

Yesterday's meeting was held in the gymnasium of Robinson High School near Fairfax City, where the teachers heard speakers from state and local teachers' associations that had organized work-to-the-rule efforts.

Mary Hatwood Futrell, past president of the Virginia Education Association, received a standing ovation and cheers of "right on!" and "amen" when she warned: "If you don't stand up right now, you cannot blame anyone else for what happened."

Florence Rosse, president of the Arlington Education Association, told the teachers that her organization's work-to-the-rule action was partly responsible for the Arlington school board's decision to give teachers 6.5 percent raises plus step increases this year. The school superintendent had recommended 3.5 percent raises.

The step increases given for length of service plus experience and advanced education that would accompany the 5.15 percent raises recommended by the school board would bring the total teacher raises to 7 percent. The teachers are insisting on 9.4 percent raises plus the step increases.

Following yesterday afternoon's meeting, the teachers drove single-file the five miles to the Massey County Office Building, where the supervisors were holding a hearing on a proposal to raise their own pay.

The teachers, many wearing black armbands with the slogan "We Care, Do You?", were prevented from entering the building by police, who said the board's meeting room was already packed. Part of a parking area was cordoned off for the teachers, loudspeakers set up, and the teachers, standing in a chilly rain, cheered each time someone testified against giving the supervisors a pay raise.

Following a 90-minute hearing, the supervisors postponed action on their own raises. They heard testimony from 11 speakers, only three of whom favored the proposed raises.

The shouts and chants of the massed teachers outside were audible in the board room and occasionally interrupted a speaker. One of the chants was: "Five point one five isn't enough to keep us alive."

The pay proposal would raise the salaries of supervisors from $15,000 to $22,000 a year and the chairman's salary from $15,000 to $25,000 a year over the next four years. The raises would start with the board of supervisors to be elected this fall and take office next January.

Conformance with President Carter's wage guidelines was urged by several speakers, among them David Johnson, an associate of White House inflation fighter Alfred Kahn.

Board chairman John F. Herrity (R) said, "I think the board is committed to no increase that will exceed the presidential guidelines" and that he supports a raise "less than the guidelines."

Following the hearing, Herrity and board Vice Chairman Martha V. Pennino (D-Centreville) told a reporter they consider it unlikely that the board will approve teacher raises greater than the 5.15 percent that is also proposed for other county employes.