A commission appointed by Fairfax County School Superintendent Robert R. Spillane yesterday recommended a radical overhaul in the county's teacher salary structure so that pay is based partly on merit -- including how well students perform.

The 12-member panel also urged the county to provide an overall increase in teacher salaries, boosting them to a level equal to that of similar professions. But the panel could not agree on an amount -- leaving that politically delicate issue up to school officials, who begin talks on 1987 salaries this week with the county's two teacher groups.

Angry county teachers have been staging a work slowdown since December to protest the increase they got beginning July 1. In rejecting their pleas for more money, School Board members and county supervisors had said they would await the commission's report.

Spillane yesterday called the report "a springboard to action" and said he would seek speedy School Board approval of its recommendations. School Board Chairman Mary E. Collier said the report would "set new directions for Fairfax County," although she said developing an acceptable merit pay system would take several years.

Members of the county Board of Supervisors, which appropriates funds for the school budget, appeared to be favorably impressed by the report although Supervisor T. Farrell Egge (R-Mount Vernon) said he was a "little disappointed" by the lack of specific salary recommendations.

Supervisor Thomas M. Davis (R-Mason) said it was a "good report" that probably will prompt supervisors to approve increases in teacher pay over several years. He would not estimate how much pay would be increased, but threw cold water on a recent national report that recommended that veteran teachers earn up to $72,000 a year. Fairfax's top teacher pay is $47,965.

The heads of the county's two teacher groups -- one of whom served on the commission -- expressed strong reservations about some aspects of the merit pay proposal.

Fairfax Education Association President Donna Caudill, a member of the panel, said it is up to Spillane "to use this report to gain a level of respect and credibility with teachers, which so far eluded him."

Commission Chairman Rodney F. Page, a former county School Board chairman, said the report's central conclusion is that the county "is prepared to pay more for education, for teachers, if there is quality control and accountability."

The "heart of the report," Page said, is its recommendation to redesign the teacher salary structure. As a first step, the commission endorsed a pilot program of performance evaluation to be tried at eight schools next year. Teachers at the schools will be rated on a five-point scale ranging from ineffective to exemplary.

The commission said there should be four levels of teacher: beginning interns on annual contracts, "career I" teachers with continuing contracts, "career II" teachers with additional responsibility and "master teachers" who would help train other instructors, write curriculum and develop policy.

All teachers above the intern level could earn bonuses for outstanding performance. One way to measure effective teaching would be student performance, the commission said. Teachers should have a voice in evaluating fellow teachers and in designing performance standards, the report said.

The commission said the county needs to do more to help marginal teachers and fire ineffective ones. Without recommending it, the commission said the county should consider eliminating annual salary increases for teachers judged incompetent -- a proposal quickly endorsed by Collier.

Caudill endorsed the commission report overall, but said the panel should have recommended significant across-the-board pay increases before endorsing an untried pay-for-performance system.

Rick Nelson, president of the smaller Fairfax County Federation of Teachers, said his group is not against merit pay but opposes trying out the plan in eight schools next year because it gives the principal, and not a committee that includes teachers, the final say over ratings.

Fairfax County teacher salaries next year will begin at $20,000, with the average teacher making $31,412. Salaries are among the highest in the region.

This year's increase included a 4 percent cost-of-living raise and additional money for beginning and veteran teachers, as well as annual step raises.

Spillane named the commission in January to report on the best ways to attract, train and hold good teachers. In addition to Caudill and Page, its members included business representatives, educators, the state secretary of administration, the president of the state PTA and a publisher of education newsletters.