James Carleton was upset when his 12-year-old daughter told him she'd accessed pornography at school. A printout of her internet usage record confirmed it, he said: The Rundlett Middle School student had visited SexyBoys.com more than 20 times.

But Carleton, of Concord, said he wasn't just angry at his daughter's adolescent curiosity. He also blamed the school.

"There's a false sense of hope that the school has a Fort Knox-type of security,"Carleton said. "And they don't."

Every school district in the state that receives federal technology grant money, including Concord, must have an internet filter to keep bad sites out and let educational sites in, a condition of the 4-year-old No Child Left Behind law. The type of filter isn't regulated, and it varies district to district.

But no matter how schools choose to patrol the web, those on the front lines say it's nearly impossible to stop all the new pornography that pops up every day.

"We just can't block everything," said Steve Rothenberg, Concord's district technology coordinator. "We've developed a culture of Google-ing, and now we need to develop a culture of students who can discriminate between what's good and what's not."

The strategy used by most districts is to filter as much as possible and then teach the students to tell a teacher - right away! - if they come upon something inappropriate. In Concord, the policy is for a child to shut off her monitor and raise her hand. If she doesn't, the teacher can assume the child went there on purpose.

"The trick is figuring out if it was an accident or not," said Roy Bailey, the technology coordinator in Bow.

There are a few basic ways to filter content on the internet. Districts can block everything and then un-block certain sites they deem appropriate. Or, they can block sites by category, such as "pornography" or "games." Most often, districts subscribe to a service that chooses which sites are respectable and which aren't.

In all cases, school officials can block or un-block sites individually. For example, when Carleton told Sarah Foynes, the director of the Rundlett after-school program, that his daughter had been on SexyBoys.com, she e-mailed the district technology coordinator to tell him the site had slipped through the filter and to block it.

"We've been very proactive about seeing patterns and blocking sites," Rothenberg said.

Foynes said it's sometimes difficult for adults to catch students looking at bad sites because kids will hide them on their computer screens. But because kids sign onto school computers under their own names, school officials can track where they've been, how often they've been and for how long. Usually, that's how students are caught peeping at something unsavory.

It's also how school technology coordinators find hot-spot sites that may be popular, but not educational. Though blocking pornography is a no-brainer, finding other objectionable sites can require a learning curve. Coordinators look for spikes in traffic and then visit those sites to see whether they're okay.

Recently, many local school districts have blocked weblog outlets like MySpace.com because students were posting too much personal information. Lots of schools also forbid students or teachers from downloading big programs or files because it takes up too much bandwidth and slows the internet connection for everyone.

But overall, filtering differs district to district. While most block chatting services like AOL Instant Messenger, some, like Bow, filter out all sites relating to hate crimes, whereas Concord allows them because students may need to research subjects like the Ku Klux Klan.

The Concord Public Library, on the other hand, doesn't filter any internet content. It's computer-use guidelines prohibit viewing pornography and using the internet to harass someone, but they state that "the internet is a global entity" and the library can't censor it. While children need parental permission to surf, high school students do not.

Paul Ericsson, the adult services supervisor, said using an internet filter would create the expectation that the library could block all inappropriate content, and that's not true.

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