Students are being warned schools are actively monitoring them online - their Facebook profiles, their forum posts, their tweets - to check for misbehaviour.

It comes after reports of high school students boasting on social networking sites during the school day of wagging classes and avoiding teachers.

But schools insist they are spying on the sites to protect students from themselves, citing photos posted this week of pupils risking injury by "planking" on the crossbar of some rugby posts.

Secondary Principals' Association president Patrick Walsh said his school John Paul College in Rotorua, and most others, had network managers and teachers charged with trawling their students' social networking sites.

"From our point of view, schools have a legitimate interest in monitoring that stuff," he said yesterday.

"We tell students that if it's up in cyberspace it's public and there for everyone to see. It's one of those things. You should be happy to say it to somebody's face and if you're not, you should think twice about putting it on Facebook."

Walsh said students often uploaded photographs of themselves in school uniform online and, if they were depicted behaving criminally or badly, it could negatively affect the school's "good name and reputation".

Other behaviour such as bullying and wagging school could also be picked up by monitoring sites such as Facebook, Twitter or YouTube.

"Despite the fact that this is at first an external event outside of school, they often erupt into problems at school and the school is forced to deal with it."

He said other students and members of the school community would also often report bad online behaviour.

Walsh said parents were often completely unaware of their child's online activity.

He said the most recent online threat was boasting about "planking", a new craze in which young people photograph themselves balancing horizontally in precarious places.

"There have already been a number of reported injuries. The other day we had a student doing it on the crossbar of a rugby goal post - that's almost a 4m fall.

"If it's not stopped we're going to end up with students with serious injuries and fatalities."

Walsh said some students had "misgivings" about schools checking up on them online but the monitoring was "universally supported by the public, staff and parents".

"We're not trying to pry into students' private affairs - we don't intervene unless they're doing something wrong."

Kevin McCormack, spokesman for the New Zealand Council of Civil Liberties, said schools needed to make it "absolutely clear" to students and their parents that their online presence would be monitored.