Websites that allow pupils to anonymously attack their teachers' working practices should be better controlled, UCAC delegates agreed.
The Welsh medium teachers' union passed a motion at last weekend's conference and now plans to call on the Assembly government to take action against such sites.
Pupils can leave comments about teachers and grade their lessons and teaching skills on a number of websites, but it was the controversial Ratemyteachers.com that attracted most anger from delegates.
The free American-based site allows pupils to rate teachers on how easy their lessons are, as well as their helpfulness, clarity and popularity. Teachers then receive either a smiley or a sad face to represent their ratings.
Hundreds of Welsh schools are featured on the site, including some that have closed down.
Although the site boasts strict guidelines and says comments should be limited to teaching, many teachers feel angry because they have no way of responding to the criticisms, some of which they claim are untrue and malicious.
Roger Williams, a permanent supply teacher at Ysgol Gyfun Gymraeg Bryn Tawe in Swansea, proposed the conference resolution calling for the government to take action against the sites.
"My name was on the site relating to a school that I had left several years before. One comment said I was 'unhelpful', which I found ridiculous," he said.
Rebecca Williams, UCAC's policy officer, said: "Our members feel really strongly about this, but it's a difficult one to do anything about.
"Pupils can put up whatever they want about teachers and it's getting people down."
Ratemyteachers.com said 60 per cent of its ratings were positive and its administrators reserve the right to delete comments and ratings.