Warning, students: Turning in your older sister or brother’s term paper is becoming a much riskier undertaking.

A growing number of New York City schools are asking students to submit their papers through Turnitin.com, a service that compares students’ papers against everything on the Internet and a database of more than 15 million student papers.

In effort to curb plagiarism and emphasize the importance of original work, private schools like Xavier and Riverdale and top public schools like Stuyvesant and Midwood are among the 278 high schools statewide, including 52 in the city, that subscribe to the service.

Within a few minutes, papers are scanned against every known Internet source and every other paper that has ever been submitted to the site. The program highlights passages in the paper and identifies its original source, in addition to giving it an overall originality rating.

About a third of the papers submitted to the site are found to be more than 25% unoriginal, the CEO of iParadigms, John Barrie, said. His company owns Turnitin.

At Midwood in Brooklyn, which attracts students from across the city with its nationally acclaimed programs in science and humanities, administrators subscribed to Turnitin after they saw a television special about cheating several years ago that mentioned the software.

“I had one student who wrote a paper and half of it was copied from a kid from the year before,” a history teacher at Midwood, Jeff Schneider, said about an assignment on Frederick Douglass.

Mr. Schneider is one of about 17 teachers at the school who require students to submit papers through the site. He also asks them to submit homework assignments the same way so he can make sure they are not copying off one another.

He said the software has cut down “enormously” on cheating.

At the school orientation, new students are shown a seven-minute clip from the “60 Minutes” show on cheating and warned about not properly attributing material.

According to the chancellor’s discipline code, punishment for “scholastic dishonesty,” including cheating, plagiarizing, and colluding, can range from a student-teacher conference to a one year suspension.

City schools that use the service pay for it with their own money. While school districts in some cities, such as Seattle, have purchased the service for use at all its schools, New York City’s Department of Education said it has no plans to pay for Turnitin for the city’s 300,000 public high school students.

Access to the site costs about 80 cents a student.

Stuyvesant High School just renewed its contract with Turnitin for about $3,500, its student newspaper reported.

“I was so motivated to use it because I was reading some of the research papers that were turned in and they went from one writing style to another – that really upset me,” the assistant principal of mathematics at Stuyvesant, Daniel Jaye, said.

He said about 20% of teachers use the program. Many students interviewed outside the school yesterday said they had never heard of it.

Students at Stuyvesant submit about 300 math research papers every year to major national competitions like the Intel Science Talent Search and Siemens Westinghouse Competition, according to Mr. Jaye who said it’s crucial to make sure that those all include proper citations.

“We subscribed to it as a teaching tool – we never made it a gotcha tool,” Mr. Jaye said.

The founder of the California-based company, Mr. Barrie, came up with the idea while teaching at the University of California at Berkeley while finishing his Ph.D. in neurobiology.

“You can have beautiful glossy books, you can have the best chalk and the cleanest schools, but if your students aren’t even doing the work in the first place, none of that matters,” Mr. Barrie said in a telephone interview. “We’re just like a referee on the football field – we’re not making a judgment call, we’re just making sure that everybody is playing by the same set of rules.”

The Horace Mann School in the Riverdale section of the Bronx started using the site after a plagiarism incident several years ago, but soon canceled its subscription because most teachers weren’t using it.

“Teachers are checking sources and comparing them with other kids’ papers and have an instinct of when kids are writing way outside their level, it’s the way teachers have been doing it forever,” the school’s director of technology, Adam Kenner, said.

The president of the United Federation of Teachers, Randi Weingarten, said there was nothing wrong with Turnitin as long as teachers can decide whether to use it.

“In this age of instant communication where information is just a click away, teachers who safeguard against cheating and plagiarism are helping students enormously,” Ms. Weingarten said.

Turnitin now has 5,000 subscribers in more than 85 countries including all the universities in Britain and all high schools in Singapore.

The site is updated at a rate of 60 million Web pages a day and retains every page it downloads. About 50,000 to 60,000 student papers are added every day.

Most students interviewed yesterday said the anti-plagiarism software didn’t bother them.

“I don’t think Stuyvesant students in general plagiarize that much,” a junior, Kathleen Kim, said. “Or if they do, they’re not just cutting and pasting – they do it intelligently.”