Each day RateMyTeachers. com receives more than 6,000 new gradings on US and Canadian teachers by students, who award marks for helpfulness, clarity of style and how easy they make their classes.
Although the site has been banned in 200 schools, the comments are predominantly positive, often effusive.
"You're the best teacher I ever had, please adopt me," one former pupil writes, while another states: "If school was the solar system, Mr Gag would be the sun."
In possibly the most inexplicable entry, a reviewer enthuses: "Mr Kiel rules. One time he chopped a snake in half with a shovel. Then he chopped it up more and a half-digested frog popped out. Then he solved 342 equations."
The website, launched two years ago, now features 2.4 million ratings covering 380,000 teachers at 22,135 middle and high schools.
Website co-founder Tim Davis, a special education teacher in Bakersfield, California, said: "Some teachers thank us, others threaten lawsuits, though we've not had any. There's generally nothing offensive or unfair."
The tone of the students' messages is in contrast to those left on UK websites.
Last year retired teacher James Murray won pound;1,250 in a libel case after an ex-pupil claimed on the Friends Reunited site that he had been sacked for rude remarks about girls.
Other comments about teachers since removed from the UK website include "poisonous little reptile with green socks and a stunning string vest collection" and "blatant homosexual who took out his frustrations on his pupils".
RateMyTeachers.com succeeds in keeping profanities and libellous remarks to a minimum because it is policed by 1,600 volunteers.
However, some personal comments still slip through. "Makes Godzilla look like the tooth fairy" one pupil observes, while another says: "There's a line between marine boot camp and PE. I don't think she's found it yet."