Fairfax County school personnel officials should have checked former Redskins running back Clarence Harmon's background more thoroughly before hiring him as an aide at Langley High School, School Board Chairman Mary E. Collier said yesterday.

Harmon, who resigned his job Sept. 19 and later asked to retract his resignation, will not be allowed to come back, School Superintendent Robert R. Spillane announced late Thursday night.

"From my perspective, his best interest and the school's best interest are mutually served by him not returning," Spillane said after briefing the School Board in a closed session about his decision.

Harmon, 29, resigned his $9,000-a-year job after it was disclosed that officials did not know of his involvement in a well-publicized drug case in Texas. Harmon also was a volunteer football coach at the school.

After Langley students and the parent-run Booster Club voiced their support, saying the drug case was common knowledge at the school when he was hired, Harmon sought to withdraw his resignation. Spillane since has received a report on Harmon's background and met with him Wednesday, officials said.

The Harmon case, coming on the heels of the arrest of former school psychologist Arthur S. Pomerantz on child sex abuse charges, raised anew the issue of background checks of school employes. Spillane has promised more thorough checks of school system employes and more aggressive investigations of complaints.

Harmon pleaded guilty to a drug possession charge in Texas in 1983, was fined $5,000 and put on probation for two to 10 years. Under Texas law, he is not considered to have a criminal record.

Harmon had no comment on the decision not to rehire him, but when the circumstances of his hiring were disclosed, he and his supporters said the drug plea did not make him unfit to work with children.

Spillane said Thursday the Texas case was a factor in his decision not to rehire Harmon, but said other reasons, which he would not disclose, also played a part. "We knew enough to know there are risks and wanted to be cautious," Collier said.

Collier, although not sharply critical of personnel officials for their conduct in the case, said, "I wish maybe a closer look had been made before he was hired . . . . It's a question of being a little more thorough."

Asked whether the schools learned any lessons from the case, she said the message had gone out to scrutinize all employes who could be "role models for children," not just teachers.

"Perhaps the lesson is . . . that the philosophy of this system will apply to every category," Collier said. "Every person who works with children will be and should be closely scrutinized."