Nearly a million teachers at 7,500 schools are listed on the site, with staff given an "overall quality" rating based on their popularity, clarity and helpfulness.
While most comments are positive, some random negative views include: "An irritating harlot"; "She should be locked up in Belmarsh (prison)" and "Nice bloke, can't teach."
The site's founder Michael Husey said 70 percent of the comments were positive and teachers find the feedback useful.
All postings are read before being published. Any that break the site's rules, which include restrictions on bad language, appearance or sexuality, are deleted.
"I know it helps teachers," he said. "We get emails back all the time from teachers ... who are using the Web site to bring them closer to their students and create mutual respect.
"They (unions) are attacking things that undermine their power structure. They aren't adjusting too well to the information age."