The Fairfax County School Board approved an evaluation program last night that will include hundreds of librarians, counselors and other school employes in the multimillion-dollar merit pay plan for teachers.

The board's unanimous vote came after brief discussion. One issue raised by some critics in the past -- that the costly plan is designed to keep good teachers in the classroom rather than to reward others -- was never raised.

The on-the-job evaluation system will be tried at 15 schools in the coming school year and but eventually expanded to all county schools. It will cover librarians, counselors, resource teachers, reading teachers and special education employes.

The evaluation program will be wrapped into the Washington area's first merit pay plan for teachers, under which all salary increases will be based on on-the-job evaluations by the 1989-90 school year.

The librarians, counselors and others already have been promised a total of 30 percent in salary increases over the next three years, the same that teachers will receive, in return for agreeing to stricter evaluations. The evaluation plan for the employes who are not classroom teachers would rate them not only on their job performance with students but also on their management, administrative or nonclassroom skills.

The schools involved are Cameron, Cardinal Forest, Little Run, Spring Hill, Brookfield, Groveton, Forest Edge and Timber Lane elementary; Lanier, Poe, Franklin and Key Intermediate; Edison and Langley High; and Lake Braddock Secondary.