In the Baltimore County Public Schools system, administrators and teachers often encourage parents to play an active role in their children's education by visiting school grounds.

But when a parent is also a registered sex offender, the school limits when and how the parent can visit a school.

The effectiveness, however, of that maneuver is in doubt because school officials don't always know who is a sex offender and who isn't, some parents said.

Such was the case at Westchester Elementary School in Catonsville, where a school father who is also on the Maryland Sex Offender Registry was seen unaccompanied on school grounds in late February.

When the man's presence on school grounds was raised at a Feb. 27 Parent Teacher Association meeting, Principal Peggy DeCrispino said she had learned of the man's status as a sex offender just two days earlier, when another parent at the school brought the matter to her attention.

Shortly after learning the information, DeCrispino said, she notified the man, whom she would not identify, of school system policy.

That policy requires the parent to send a written request to his or her child's principal asking for permission to enter school grounds, said Kara Calder, a school system spokeswoman.

If the principal grants permission, Calder added, the parent must be escorted by a school staff member at all times on school grounds.

Many parents at the PTA meeting defended DeCrispino's handling of the situation.

They said it was impossible for school administrators to screen every parent who comes onto school property.

They also said all school volunteers are subject to background checks, and all parents entering the building must sign in at the front desk.

But other parents disagreed.

They noted the man could have been on the school grounds or in the school, at anytime since September without the administration's knowledge that he was a sex offender.

One parent said the administration had failed to adequately protect students, and needed to be more vigilant.

Another said the fact that the school was unaware of the man's status on the registry since 2006 -- and that the father was unaware of the school's policy on sex offenders -- shows a loophole in the school system's sex offender policy.

"I think that the issue needs more attention, and clearly needs more legislation," she said.

Charles Waechter, a longtime Arbutus-based criminal defense attorney who handles sexual offense cases, said judges and prosecutors often require that convicted sexual offenders have no contact with individuals under a certain age as part of their probation.

But that is not always the case.

Waechter said he advises his clients to inform their children's schools that they are a registered sex offender, but he has never seen a judge or a prosecutor require such a disclosure as part of an offender's probation.

According to Baltimore County State's Attorney Scott Shellenberger, a judge "can really, as a condition of probation, put just about anything they want" as a condition.

But Shellenberger, like Waechter, said he could not recall a situation in which a parent was required to notify a school of his or her offender status as a condition of probation.

Charles Herndon, a school system spokesman, said he was unsure whether county schools took it upon themselves to inform parents of the school system's sex offender policy either.

He said he also did not know whether schools routinely included a disclaimer announcing the policy in literature that is sent home with students at the beginning of each school year.

The issue of school administrators being unaware that a parent at their school is a registered sex offender is not unheard of.

While principals at many area elementary schools said they had never had to deal with such a situation, Westowne Elementary School Principal Patricia Vogel said her administration "has had to be mindful of not having a parent (alone) in the building," she said in referring to a parent who is a registered sex offender.

Vogel said she has her staff check the state's online offender registry periodically to keep the faces there fresh in their minds.

"It's just being mindful of the situation and knowing that you would then escort that parent," Vogel said.

Many other principals in the area declined to comment for this story.

There are 39 sex offenders living in Westchester's 21228 ZIP code and 45 in the neighboring 21227 area, which includes Arbutus, Lansdowne and Halethorpe.

According to Nancy McBride, national safety director for the National Center for Missing and Exploited Children, statistics show that children are vulnerable to sexual predators on their way to and from school, and the areas around schools are attractive to sexual offenders.

"Offenders look for access and opportunity, and what better place than a school and what better time than when kids are going to and from?" McBride said.

"Schools need to be vigilant and need to know that this is a time when children may be more vulnerable and more at risk."

Dr. Fred Berlin, director of the National Institute for the Study, Prevention, and Treatment of Sexual Trauma at the Johns Hopkins University School of Medicine in Baltimore, said he agrees that schools must be vigilant.

But he also warns against parents and administrators looking at the state's sex offender registry as "a broad brush that lumps all sex offenders together, that says they're all equally dangerous."

People should trust that judges and prosecutors are "making thoughtful decisions" when determining conditions of probation, he said.

Some parents would rather trust technology to help safeguard their children.

One example is Raptorware, a Houston-based company that has provided "visitor management systems" to 5,000 schools in 40 states since 2003.

The system scans every visitor's driver's license and checks the information against a massive, national database maintained by Raptorware, said Allan Measom, its president and chief executive officer.

The software can be updated so that an offender who is following the school's policies doesn't set off the system every time he or she visits, Measom said.

A similar system is being tested by the Frederick County school system.

Measom said any school policy to handle sex offenders other than an identity scan provides little other than a "false sense of security."