Teachers have been warned not to add pupils as "friends" on social networking sites like Facebook, as it might blur the boundaries between school staff's professional and private lives.
At the National Union of Teachers' annual conference, teachers were told that pupils are getting access to potentially embarrassing information about teachers on their Facebook pages.
And that headteachers and school governors are increasingly using information posted on social networking sites to screen candidates for jobs.
Teachers were also warned about the rise of a new site, Formspring, which enables users to post comments and questions without identifying themselves.
"The line between private life and professional life is blurred now because of social media," the Guardian quoted Karl Hopwood, an Internet safety consultant and former headteacher, as telling the NUT fringe meeting.
He urged the audience to watch for unguarded comments that could have damaging consequences.
The NUT has recently issued guidance-warning teachers not to befriend pupils on social networking sites, and to let school management know if they befriend parents or ex-pupils.
Hopwood urged teachers to be more sophisticated in their Internet use.
"Don't friend pupils on social networking sites. Set up a group to link up with the orchestra or the rugby team," he said.
Amanda Brown, NUT assistant secretary for employment, conditions and rights, said there was "definite concern" that there had been an increase in schools looking for information online.
In the public sector there are rigorous appointment procedures, she said.
"If that's going to be undermined by headteachers and governors going on to Google to see what else they can find then that would be a problem," she said.
A website used by schoolchildren to spread anonymous gossip was closed down earlier this year, after what its owners said was abuse by a "minority of irresponsible people".
Little Gossip, which started in November, had been criticised for failing to remove schools from a list of institutions users could gossip about.
Hopwood expressed concern about the potential misuse of the Formspring site, which he said had a "much bigger user base" than Little Gossip.
"They think they can say anything on Formspring, but their online reputation can be damaged just as quickly as yours or mine can," he said.
However he also praised social networking sites as a space for sharing positive information, giving the example of a memorial page for a teacher on Facebook.
"Some of the stuff on there was wonderful," he added.