GUILDERLAND CENTER, N.Y. -- Students have gossiped about teachers since lessons were taught in log cabin schoolhouses. They trade stories in the cafeteria and locker room _ always out of the earshot of instructors.

Today, tech-savvy students are going a digital step further and grading teachers over the Internet by clicking on RateMyTeachers.com, prompting some schools to block the site and supporters to protest what they see as an attack on the right to free speech.

More than 500 schools and districts in the United States and Canada have Internet filters that ban the teacher-rating site _ along with those containing pornography, racism and other objectionable content _ from school computers.

"They're putting us in some pretty interesting company, saying we're quite a danger to education when I don't think that's clearly the case," said Michael Hussey, who co-founded the site with two teachers in 2001.

Schools that prohibit teachers and students from logging on the site during school hours contend it has no educational value and reject claims that they are trying to shield teachers from negative comments.

"If a student wants to visit that Web site, they have to do it on their own time outside of school and that's their right to do so. We're not infringing on their right," said Gregory Aidala, superintendent of the Guilderland Central School District.

Here's how the ratings work: Students anonymously judge their teachers on a 1 to 5 scale in categories like "easiness," "helpfulness," and "clarity." Students can also type a sentence or two explaining their grade. Teachers with favorable ratings have a smiley face in sunglasses next to their names.

To prevent students from taking a cheap shot at teachers, ratings are vetted by an army of about 1,800 student volunteers, known as "admins," who shift through responses for potentially libelous material. Profane language, threats or comments about a teacher's sexiness or physical appearance are tossed out.

Tian "Max" Xia, a 16-year-old admin, said the majority of reviews he receives are positive. Of the hundreds of postings a month by Guilderland High School students, he deletes about 3 percent that contain bad language or physical attributes of teachers.

Michael Williams, an 11th grader at Guilderland High School, believes it may not be in the best interest for schools to block the teacher-rating site since educators can use the marks to refine their teaching methods.

"The site allows teachers, students and school administrators to understand the way teachers work, the way they tick," said Williams, who rated more than a dozen teachers.

Since RateMyTeachers.com's debut, students have posted more than 3 million ratings for more than 550,000 teachers at 30,000 middle and high schools in North America. Reports of schools blocking the site started trickling in late last year, with new reports of a "handful" of schools each week, Hussey said.

In contrast, a sister site that lets college students give marks to professors has been blocked on only five campuses nationwide.

"I think colleges tend to be more respectful of the First Amendment and a little less oppressive of students," said John Swapceinski, founder of RateMyProfessors.com, who also is a partner of the teacher-rating site.

Cindy Warner, a spokeswoman for the Shelby County Schools in Alabama, disagrees. The district blocked the site after receiving a complaint from a high school principal last spring that students were using class time to post reviews.

"We are not saying that they don't have a right to go to that Web site," she said. "We are only saying that they need to do it on their own free time, from their home computers."

In some cases, Hussey has directly appealed to administrators to unblock the site. Only 10 schools or districts reversed their decision so far.

Teachers should be evaluated by professionals and not by a Web site where students judge teachers based on a "popularity contest," said John See, a spokesman for the American Federation of Teachers.

Matthew Green, who teaches technology education at Upper Darby High School in Drexel Hill, Pa., relishes feedback from his students. Before online rating existed, Green used to ask his students to anonymously fill out a survey at the end of the year about his class. Now he encourages students at the school, which has a software filter that automatically blocks the site, to post ratings from home.