As schools across the country go online, the question of what constitutes acceptable use is in dispute.
There are no central statistics to indicate how many teachers have been disciplined for misuse of the Internet but unions are predicting that the caseload will increase.
There is a grey area between using school facilities to book a holiday and downloading "inappropriate", but not illegal, images.
Susan Marsh, National Governors' Council treasurer, said: "Someone shopping on the Internet is like someone spending copious amounts of time on the telephone. It might be forgiven on the odd occasion but if it is excessive it should be brought to the teacher's attention.
"Accessing inappropriate sites is more serious because teachers have a duty of care and parents have a right to expect certain standards.
"Just as you would not expect a teacher to wonder around with an unsavoury magazine under their arm, as a governor I would take a dim view of a teacher accessing those kinds of sites."
The unions take the same line. Kay Jenkins, NUT legal services assistant secretary, said: "Serious cases, which are very rare, would lead to dismissal and we would not take issue with that."
Unions and other teachers' associations are now calling for clarification on where to draw the line.
Gareth James of the National Association of Head Teachers said: "The lack of guidelines means one or two people have fallen foul. There are uncertainties surrounding issues like sending e-mails for example."
It is also difficult to prove that a teacher has intended to access a site in the fast-changing world of cyberspace. Schools can limit access to sites, for example by choosing Internet service providers who offer a "walled garden" of pre-selected software which filters out unsuitable material. However, even with such filters some unsuitable material slips through.
While searching for a site on primary sex education policy the NGC treasurer found herself invited to join an adult club in California. And one user was caught out when a search into the Yeti legend led to a pornographic site for people with a big foot fetish.
Midlands NUT has dealt with five cases of inappropriate use of the Internet in the past three years. Two teachers said they were seeking out the sites in order to protect pupils from them. Another claimed to have accessed the site accidentally.
A number of local authorities are now in the early stages of producing guidelines.
In the wake of a recent case where a headteacher was convicted of downloading child pornography, West Sussex County Council has warned it will adopt a hard line where misuse is discovered in school.
Shropshire has sent a 10-page code of practice to all schools and Derbyshire is in the early stages of formulating a policy.