The Fairfax County teachers union is planning, for the first time in five years, a "work-to-the-rule" action to protest the 4 percent pay raise offered last month to all county employes by the Board of Supervisors.

Donna Caudill, president of the Fairfax Education Association, announced yesterday that the organization has asked its 6,300 members to work strictly by the contract starting Jan. 7.

That means spending exactly 7 1/2 hours on the job and eliminating all extra duties for which teachers are not paid, including correcting homework assignments in the evening, helping students during lunch break and chaperoning field trips, Caudill said.

The FEA is asking for a 7 percent pay raise, and Caudill said teachers will begin an intense lobbying effort on Monday to persuade the Board of Supervisors that its demand is reasonable.

"We are putting maximum pressure on the superintendent and the School Board . . . to get them to go with us to the Board of Supervisors," Caudill said.

The Board of Supervisors, which appoints the School Board and ultimately approves the school budget, has in the last three years tied the school spending increase to projected tax revenues.

Caudill said the board raised the amount by which the county budget can be increased from 7.95 percent last year to 9.11 percent this year, but is seeking to cut the pay raises of county employes from 5 percent last year to 4 percent this year.

The supervisors, she said, "sell Fairfax County on the fact that we have excellent schools, but they don't reward teachers for the excellence."

FEA members acknowledged, however, that their last "work-to-the-rule" action, which began in the fall of 1979 and lasted through the spring, did not achieve its goal. At that time, the union was forced to accept a 5 percent raise, instead of the 7 percent it had asked for, according to FEA executive director Barby Halstead.

Since 1977, when the State Supreme Court ruled that collective bargaining was illegal for all public employes, teachers have had little power in salary matters. The current negotiations, which are called "discussions," began on Sept. 17 and broke off Nov. 16 without an agreement.

Mary Collier, chairman of the School Board, said yesterday that she had no doubt teachers would continue to work "to ensure that students get a quality education."

According to a study released last month by the Virginia Education Association, Fairfax County's average salary of $25,340 is the fourth largest in the state. The city of Alexandria ranks first, with average pay of $28,461, followed by Falls Church with $27,202 and Arlington with $26,521.