Every school in the UK should have rules about how teachers use Facebook, a teaching union has said.
The National Association of Head Teachers (NAHT) said new teachers are having problems because of using social networking sites.
The issues include staff being told off for what they have put online or being embarrassed by pupils who discover too much about their private life.
The NAHT wants every school to set out exactly what is and isn't allowed.
"Schools need to get up there and state things clearly," said Russell Hobby of the NAHT.
"Otherwise teachers may be disciplined unfairly," he added.
Mr Hobby also said that many new teachers are caught out by how much they need to change what they do online when they start work.
"You're used to a much more relaxed environment at university where what you say and do doesn't matter so much," he said.
"Then you're suddenly into a place where you've got people expecting you to be role models," he continued.
Sue, who didn't want her full name to be revealed, is a head teacher in London.
She told Newsbeat that she's seen a big increase in the number of young teachers getting themselves into trouble because of low privacy settings.
She recalled one case where pupils found the profile of their new teacher and managed to access some of her photos.
They printed them and made comments about the pictures to her in class.
"It's not that teachers are totally unaware," she said. "Often they just don't understand how serious this sort of thing can be."
"Facebook is really tricky," said teacher Dupe Oyeleke. "There are no clear guidelines as to what you can and can't put up.
"It is your personal life, you do your own thing, and it can be very difficult to know where to draw the line."
Dupe has never accepted friend requests from pupils but knows colleagues who have.
"Not everyone has the best judgement and they get into trouble.
"Things like that affect you, especially as a teacher, where one black mark could impact your career," she said.
"I do look up my teachers," 17-year-old Ellie Steel told Newsbeat.
"I like to check out what they are getting up to at the weekend to get some gossip for when I get back to school."
Joe Mercer, also 17, also admits to trying to find out about his teachers on Facebook.
"The stuff we're after is probably the kind of things they do not want us knowing, mainly photos really," he said.
Ellie agrees: "Definitely the drunk photos - which we do manage to find."
Both Ellie and Joe reckoned their classmates would use these things against their teachers:
"Maybe you'd get the odd comment if they were having a go at you," said Ellie.