Debra Royals used to give a short daily quiz to her students at Chantilly High School to make sure they did their homework. But she dropped that when Fairfax County teachers began a job slowdown this month.

The 32-year-old Spanish teacher said she does not have time to grade the daily quizzes during school, and the "work to the rule" action by the county's two teacher associations calls for refusing unpaid labor beyond the 7 1/2 hours a day required in the teachers' contract. Instead, she will test only every two or three weeks, at the end of each chapter in the textbook.

"When that one test comes up for the chapter, they're probably not going to do as well because they haven't been forced to study," Royals said. "In the long run, you'll start seeing results" in lower academic achievement.

The job action is a protest of Superintendent Robert R. Spillane's proposed pay package for the 1986-87 school year, which teachers say is inadequate. The Fairfax Education Association estimates that 90 percent of the 8,000 teachers in the Washington area's largest school system are participating in the protest to some extent.

It is the strongest protest by Fairfax County teachers since a similar job action that lasted from April 1979 into the following year and won teachers a substantial raise.

Students at a half-dozen high schools and intermediate schools staged sit-in demonstrations to back their teachers' demands in recent days before teachers made it clear they preferred less disruptive forms of support. Some students have organized petition and letter-writing drives and have signed up to speak at February's School Board budget hearings. But even those students concede that some of their friends joined sit-ins only to get out of class.

The effects of the teacher job action range from canceled dances to slower grading of homework or tests. The impact varies from school to school, from barely perceptible to slightly painful, according to principals, teachers, students and parents.

"At 3 o'clock, things get quiet," said Annandale High School principal James G. Finch. "Other than that, we haven't had any noticeable change." But Ted Miller, a senior at South Lakes High School in Reston, said, "It's hurting our extracurriculars, and it takes years to get our papers back."

Especially at the county's 23 high schools, many after-school clubs, dances and other activities have been scrapped or rescheduled for school hours. Canceled dances were one reason some students at Hayfield Secondary School in Alexandria gave for their sit-in.

Franklin Intermediate School in Chantilly dropped plans for a science fair in February because it would have been held on a weekend, when teachers would not work, said PTA President Cynthia Lauster. A student entertainment program at Clermont Elementary School in a section of the county near Alexandria was moved from an evening to an afternoon, which PTA official Janet Smith said meant that fewer parents could attend. At Chantilly High, an "International Night" program became "International Day."

In some schools, teachers have substituted multiple-choice tests for essay exams because they take less time to grade. Miller said his South Lakes math teacher no longer gives partial credit for the number-crunching that leads up to a test answer. Edgar McConnell, an award-winning science teacher at McLean High School, said he asks students more often these days to help him grade their work in class.

Gavin Cato, a senior at Robinson Secondary School off Braddock Road, told the School Board last week that the slowdown, which he supports, is hurting students. They are getting their homework or tests back so slowly that they do not know how they are doing in class, he said.

And although Spillane emphasized that writing college recommendations is a contract duty, some students said they have had to hustle to find a teacher willing to help. At Robinson Secondary School, said senior Beth Underwood, some students have had to rely on guidance counselors who do not know them as well as the teachers do.

But many schools report little visible impact. In some cases, activities were winding down for the holidays; Friday was the last day of school until January. In others, parents stepped in to take over for absent teachers -- chaperoning a sock hop at Robinson, for example. At Key Intermediate in Springfield, "It definitely appears that the majority of teachers are going beyond the regular hours," said PTA President Marge Byrnes.

And Spillane, who has taken a hard line, said he has heard of only a few cases in which principals have had to remind teachers "that work-to-the-rule means do what your principal asks you to do." The superintendent said he is sure that there are other situations in which teachers have "dressed down other teachers" for unprofessional conduct.

So far, most parents and students apparently back the teachers' demands. Sharon Murphy, president of the South Lakes PTA, said she has had only a few "hysterical parent complaints" from mothers and fathers concerned about such unlikely possibilities as the cancellation of graduation. "Of course we'll have graduation," she said. Several PTA presidents said they had heard from no unhappy parents.

But some said it is too early to assess the real impact. "Undoubtedly, in the long run it will affect the education our children will receive," said John Pollard, copresident of the Mount Vernon High School PTA. "Then I'm sure some parents would complain."

The County Council of PTAs, with 151 chapters composed of 50,000 members, said Spillane's pay offer was not high enough. It urged more talks among teachers, Spillane, the School Board and the County Board of Supervisors, which holds the purse strings, to arrive at a better offer.

Spillane rejected that proposal, saying the next step is for the School Board to vote on his proposal as part of its budget action Feb. 19. The budget then goes to the Board of Supervisors for a vote in April.

Spillane proposed a 4 percent cost-of-living increase -- less than half of what the teachers requested -- as well as higher starting salaries for beginning teachers and pay increases for the most experienced teachers.

New Fairfax teachers start now at $18,385, the second highest beginning salary in the Washington area. Spillane has proposed an increase to $20,000. The average teacher pay in the Fairfax system is about $29,000.

The teacher associations, which are prohibited from collective bargaining or strikes, say two-thirds of the county teachers would get nothing more than the cost-of-living increase and a grade raise under Spillane's plan.