The three mothers waiting to meet with the principal looked harmless enough, but before they were allowed into the Spring-Ford district school, they had to undergo a computerized background check.

The women gave their driver's licenses to a secretary, who scanned them against a national sex-offender database. All three were cleared and were given badges with their pictures, names, and destination printed on them.

"I think it's great," said Michele Decerio, whose son attends the fifth-and-sixth-grade school in Upper Providence, Montgomery County. "As a parent, I'm all for extra added security."

So are a number of schools that have installed the security scanners this year. Officials in these districts - including Methacton, Phoenixville, Radnor, and North Penn - describe the devices as yet another layer in an increasingly dense web of security measures in their schools, on top of cameras, 24-hour guards, and criminal background checks on employees and regular volunteers.

In South Jersey, only Lenape High School in Medford uses the scanner.

So far, parents like it, administrators say, proof that after a decade marked by the Columbine massacres, the 9/11 attacks and high-profile child sex crimes, nothing may be "too much" when it comes to child security, regardless of the cost or intrusiveness.

"I never want to be the superintendent who has to tell a parent something happened to their kid," Spring-Ford superintendent Marsha Hurda said during a demonstration of the system, installed in 13 school buildings this year at a cost of about $1,500 each.

Raptor Technologies Inc., the Houston company that makes the system, said it's been installed in 6,000 schools nationwide since it was created seven years ago as a visitor-management system for Enron Corp.

In addition to flagging sex offenders, the system manages the flow of visitors and keeps track of their whereabouts. It also alerts staff to parents who are involved in custody disputes or subject to restraining orders.

Visitors who clear the sex-offender registry are sent on their way and told to check out when they leave.

But if there's a match, school administrators, and sometimes police, are notified. If there's a legitimate reason for the visit, such as a teacher conference, the visitor is allowed in with an escort. Participating in other school functions, such as volunteering in class or chaperoning a field trip, is generally not allowed.

"Even if you are a parent and are on one of these Megan's Law lists, you are not allowed to come in and freely visit our buildings," said Methacton School District superintendent Terry Quinn.

Or, as Julie Mullin, a Spring-Ford Area School District board member and mother of seventh-grade twins, said, "I don't want them reading to my kid."

Interest in the scanners, and a half-dozen others from other companies, comes amid a growing national debate over whether the ever-swelling lists of registered sex offenders - 674,000 by the last estimate - are really that helpful or simply provide a false sense of security.

The company says its software identified 1,700 convicted sex offenders at schools last year.

Civil libertarians note that from 80 to 90 percent of people who victimize children are someone the children already know, such as a parent, family friend or coach.

"It certainly can be helpful," Kristen Anderson, a spokeswoman for the National Center for Missing and Exploited Children, said of the technology, "but it's no substitute for good common-sense protocols and parenting."

Lauren Taylor, executive director of the Pennsylvania Sexual Offenders' Assessment Board, said the idea that a molester would wander into a school to harm a child is "a very unlikely scenario." It's more important to thoroughly vet those who come in close contact with children, such as teachers, health-care workers, and janitorial staff.

"I'm not sure the bang for the buck is there," she said of the security devices.

In Pennsylvania, about 130 municipalities have passed residency restrictions for sex violators, and many more are considering doing so. Those requirements often include bans on living or loitering near schools.

There are almost 13,000 sex offenders in Pennsylvania registered under Megan's Law, which requires them to give their whereabouts to police, who notify the public through an online registry.

"It includes all kinds of people who you have no reason to believe are a danger to kids," such as those convicted of statutory rape for having sex with an underage girlfriend, said Mary Catherine Roper, staff attorney with the American Civil Liberties Union of Pennsylvania. "I'm really kind of disturbed by this concept."

The issue recently came before the courts when an Austin, Texas, couple sued their school district, arguing that the system violated a number of their constitutional rights. In August, a U.S. District Court judge ruled in favor of the district.

While area schools say they have not tagged any sex criminals yet, there have been false-positives of visitors who have the same names as convicted felons.

Raptor founder E. Allan Measom says a school can expect a couple of those a week. "It depends, how many Smiths do they have?" he said.

It's happened twice at Radnor High School, said director of operations Leo Bernabei, noting that the school gets 32,000 visitors a year. In both cases, a staff person compared birthdays, addresses, and pictures of the offenders to the school visitors and saw they weren't the same people.

Neither person was upset about the mix-up, though "one can only imagine" the embarrassment of being confused with a sex offender, even for a few minutes, he said.

Quinn, of Methacton, in central Montgomery County, has not had any misidentifications yet, but if they do occur, he said he thinks the community will understand.

"That's a slight inconvenience," he said, "because, ultimately, we're trying to make sure our kids are safe, and how could we be knocked for that?"