The Rate My Teachers website invites pupils to give their teachers points for clarity, helpfulness, and overall quality.
Entries also include critiques of teachers such as: "Mrs Clifford was pants. She used to cry a lot, and didn't teach us maths very often."
One East Anglian graphics teacher is described as: "So stupid he got the train the wrong way and was late for class. I could learn more from a carrot."
Seven schools in the UK have already blocked the site, so that pupils cannot access it from classroom computers.
St Michael's high, in Lancashire, is among them. Liz Nicholls, head (described on the site as a "power-hungry dictator", but also "a lovely headteacher"), said: "The site is completely inappropriate.
"Pupils have multitudinous opportunities at school to give feedback, but not to give anonymous comments with no accountability whatsoever."
Comments such as "sexy" or "hot" are forbidden by the site's moderator, as are references to physical appearance. Racist comments and threats against teachers are also not allowed.
But many remarks nonetheless seem to border on the libellous. One pupil writes of his teacher: "Smacked a kid, but was still allowed back."
The NASUWT, the second largest teachers' union, is monitoring the site, and has taken legal advice. Chris Keates, general secretary, said: "We think the site is unacceptable and a nonsense.
"It doesn't add any value at all, but can be harmful to teachers' personal and professional lives. Teachers have enough challenges in the job without this."
In 2002, a member of the NASUWT was awarded pound;1,250 damages, after comments were posted on the Friends Reunited website claiming that he made rude remarks about girls, and strangled a pupil.
But Seb Haigh, of Rate My Teachers, defended his website. He said: "There's a general feeling that there isn't sufficient information about what happens in the classroom. We want to provide useful feedback for teachers.
"The only way to avoid risk is to have no comments whatsoever. But I think blocking the site is a little bit over-zealous. I could think of more important sites to block."
The website says part of its job is to expose ineffective teachers, but also warns pupils posting on the site that nasty messages will be removed.
Schools which block the site are listed on a wall of shame on the site if pupils report them.