Teachers' private lives are coming under scrutiny from heads who are "trawling" the internet in search of incriminating material, union officials have warned.
Staff who post "inappropriate" pictures online from holidays or nights out are being disciplined by school leaders for breaking the General Teaching Council for England's (GTC) code of conduct, according to the NASUWT.
Others have received informal warnings for attending a gay pride march and being involved in relationships with colleagues, the union said. It raised the issue with the GTC council, but failed to get the code revoked.
Union executive members claim the code has left teachers "subject to intense scrutiny".
"It is being held over teachers' heads like a giant sword of Damocles and is having a huge impact on their private lives," said Ron Clooney, NASUWT executive member for the south of England.
"Unscrupulous employers are hiding behind it when they want to get rid of a teacher. They are using it to try to add gravitas to allegations."
The code of conduct was introduced in October 2009, prompting concerns that it would "intrude" into teachers' private lives. It says teachers should display "standards of behaviour both inside and outside school that are appropriate given their membership of an important and responsible profession".
The NASUWT said there had been a surge in disciplinary hearings and informal action since the code was brought in. Cases include teachers being disciplined for putting pictures on Facebook of them when drunk or wearing revealing clothes.
One teacher couple who used "fetish clothing" outside of work were told they were not upholding the "moral standards" of the school, according to the union.
NASUWT executive member Mick Lyons, who serves on the GTC council, said: "It has become common for heads to trawl through the internet and use what they find to discipline teachers. Other teachers are venting their spleen about their jobs on the internet and this is rebounding on them."
NAHT general secretary Russell Hobby said the code had an "uncertain status" because of its "perceived illegitimacy" and the planned axing of the GTC in March 2012.
"We need to have a shared view about teacher conduct which everyone subscribes to, so there are no grey areas," he said. "There has to be a consensus and a debate about what is responsible behaviour. Teaching is a community role and their actions should not be at odds with that."
Despite concern about what is happening at school level, the GTC said there was no evidence of increased referrals to the council relating to non-criminal behaviour in teachers' private lives.
A spokeswoman said the council had heard only two cases relating exclusively to teachers' non-criminal behaviour outside school, and both were before the code's introduction.
"The code does not alter the threshold for disciplinary action and does not affect the way in which any cases referred - including those involving conduct outside school - are assessed," she said.
"It is only if a teacher's private behaviour were to become public in a way that was damaging to the reputation of the profession, and if that behaviour were serious enough to warrant dismissal, that the GTC would become involved."
CODE OF CONDUCT: Always on duty
The GTC code of conduct contains eight "core values" designed to "uphold public trust and confidence" in the profession.
They include demonstrating "high standards of honesty and integrity" and working as part of a "whole-school" team.
The most controversial clause asks staff to maintain high standards of behaviour inside and outside work. Unions claim the rule means teachers are "permanently on duty".