Two years ago, the arrest of a Fairfax County school psychologist on child sexual abuse charges prompted officials to promise more aggressive efforts to ensure that school employes have clean records.

But the recent discovery that a Reston elementary school assistant principal was convicted on a morals charge in 1982 in Prince William County indicates that, despite changes to tighten procedures, some problems remain.

Mark Arbeit, 33, was suspended from his job at Terraset Elementary School in late April after several students had alleged that he made obscene phone calls to them.

While investigating those charges, school officials learned that he had been convicted in Prince William County General District Court of soliciting a 15-year-old boy to pose for an obscene photograph.

The incident raises new questions about the school system's ability to screen its employes for criminal convictions in an era when national concern about child sexual abuse has heightened and the number of reported cases has soared.

Fairfax County has increased its investigation of complaints against current employes since that 1985 case, but its background checks of job applicants are no tougher than they were then, school officials acknowledge.

In the meantime, because of enactment of a tougher screening law in Maryland last year, the schools' investigator is concerned that applicants with cloudy records would apply in Virginia where screening is less strict.

In the 1985 case, psychologist Arthur Pomerantz was convicted in Fairfax County Circuit Court of five counts of engaging in sex with teen-age boys. Prince William police said they had investigated but not arrested him on similar charges in 1981, and told Fairfax school officials of their suspicions. Several colleagues also said they had voiced suspicions of Pomerantz.

Superintendent Robert R. Spillane, saying school officials should have pursued the case earlier, promised more aggressive investigation of future complaints. He established an investigative unit in the personnel department to perform better background checks, and pledged closer liaison with police departments.

Some Terraset parents fault the school system for not discovering Arbeit's conviction earlier. "I don't think there's any excuse for that," said Barbara Banks, a member of the Parent-Teacher Organization.

"I would like to know why they aren't more on the ball," said another mother, who requested that her name not be used.

Arbeit joined the school system nine years ago, before new security procedures were implemented. But Spillane said the Arbeit case offers proof of "something we already knew -- we were not getting information from surrounding jurisdictions until we asked for it a year and a half ago . . . . There's nothing we could have done."

Prince William police never notified the Fairfax school system of Arbeit's 1982 conviction, and it was only investigative work by Fairfax officials that turned it up five years later, Spillane said.

Many parents agreed. "The school system did exactly what it should have done," said Terraset PTO President Ruth Toxopeus. "If you want somebody to be upset with, Prince William County police have something to answer for."

One result of Spillane's 1985 pledge of tougher background checks and more aggressive pursuit of complaints was the hiring in February 1986 of Alan Barbee, a retired Fairfax deputy police chief.

Barbee said his main function is to investigate complaints against school system employes, ranging from alleged theft to alleged child sexual abuse. In 1986, he looked into 63 cases; 25 of the employes involved were fired or forced to resign as a result, he said. In the remaining cases, the allegations were unfounded or the evidence inconclusive, he said.

Nine of the 63 cases involved child sexual abuse, he said. In two cases, both teachers, the employes were fired, Barbee said. The other cases were unfounded or inconclusive, he said.

So far this year, Barbee said, there appear to be fewer cases, although he has not added up the numbers. Three were sex-related: A bus driver arrested in Fairfax on allegations of having sex with a minor, and two employes arrested in another jurisdiction for sex offenses not involving minors, he said. All three employes were fired.

Barbee said some of his tips have come from police departments in other jurisdictions, and overall, "We're better able to address problems than we were two years ago."

Despite Spillane's promise of better cooperation with other area police agencies, Barbee said, "There are a lot of potential fallout points. I can't say I'm confident that we receive reports of everything."

The county has failed to implement stricter background checks of job applicants, Barbee said, mainly because the Virginia General Assembly turned down its request for the necessary permission to run FBI fingerprint checks. Fairfax can check only on Virginia criminal records -- not very helpful in a transient area such as Washington.

In Maryland, job applicants in the schools and in a wide range of child-related activities must be fingerprinted and have their records checked. In the District, legislation has been proposed but not passed. Virginia has no such legislation, although the state makes it a misdemeanor for school employes to lie about convictions on job applications.

Barbee said some officials wonder whether the strict Maryland law could be hurting Virginia. "There's always a concern," he said. "How many people are applying here and not applying there because we cannot do that" fingerprinting?