Staff are told to let it go
The team behind ratemyteachers.co.uk have started up a rebuttal system where teachers are encouraged to defend themselves against the anonymous postings.
They claimed teachers are pleased with the new feature and are joining in.
Most, they said, are logging on to thank pupils for positive comments, although a number have reacted to negative feedback.
Chris Keates, general secretary of the NASUWT, said: "No teacher should dignify this invidious site by responding to anything posted. Teachers who have concerns about anything written about them should contact their union for advice."
The National Union of Teachers also criticised the site and said it would take action on behalf of its members should they be libelled.
"The vast majority of teachers are too sensible to waste their time on this site - we would rather they went and had a cup of coffee," said a spokeswoman.
The site has been running for less than six months, and already there are more than 100,000 ratings on 26,000 teachers.
The exact number of teachers who are responding to comments cannot be determined as many use the facility anonymously.