A grassroots student movement dedicated to opposing security measures taken since the terrorist attacks of September 11, 2001, is emerging at a selective Lower Manhattan high school.

Students at Stuyvesant High School this week put up posters comparing their principal, Stanley Teitel, to a fictional Harry Potter villain, Lord Voldemort, and schemes to convene a protest, sit-in, or meeting of the school’s more than 3,000 students are the talk of the hallways and subject of a new Web site.

The students describe walking into their Lower Manhattan building this fall to find a new kind of school: Among the surprises were a scanning system that now tracks students when they leave for lunch as well as when they enter the building; requirements that teachers wear identification cards on their bodies — and such a policy for students is a possibility — and a new locker system students said they fear gives administrators power to gain access to their belongings at any time.

“Since we’ve been here, they’ve just been slowly taking away our freedom,” a 17-year-old senior who lives on the Upper West Side, Maorizio Martinelli, said. “This is our fight for the student rights that we lost.”

Students said the policy changes strip them of cherished traditions, such as groups of friends picking lockers in clusters and decorating them with their own locks. Under Mr. Teitel’s new policy, the school assigns students lockers and locks.

Leaving for lunch has also become an ordeal, a 17-year-old senior, Thiviya Navaratnan, said. “He said it would be okay if we were a little late,” Ms. Navaratnan said, but in fact she’s seen friends lose privileges to eat lunch outside school if they enter even a few minutes late.

Dubbed “Kids First,” the student movement kicked off last week with a Web site, StuyWatch.com, that is the work of two students who said they are keeping their identities a mystery, even from their friends, as part of a bid to attract attention.

The tactic seems to be working. Under instructions from the founders, movement members have created posters advertising the campaign and pasted them around the school. On the heels of the postering blitz, several students who are also members of the site confronted Mr. Teitel at an after-school planning meeting yesterday that is usually attended by a small group of students, teachers, and parents. The Web site was the subject of a front-page article in the student newspaper yesterday, and as of last night it had recruited 465 registered members and 3,400 unique visitors, its founders said.

“There’s more resentment in the student body against the administration’s policy than there has been in the past,” a senior who is an editor at the student paper, Yasha Magarik, said.

Mr. Teitel said yesterday he had heard of the Web site but did not visit it and did not give its rising membership much credence, as signing up is easy to do and free. “If it cost them something — well, then I might begin to think about something,” he said.

He also defended the policies students have targeted, and the importance of security. “Let’s see, there was a little incident that occurred on 9/11. I think some short distance from my school two buildings fell down,” he said, also citing recent shootings at university campuses in Virginia and Delaware as another cause for concern. “Am I being unrealistic to try to know who’s in the building and who’s not?”

Mr. Teitel, who has been Stuyvesant’s principal since 1999, is the target of much of the students’ frustration.

The StuyWatch Web site kicked off with postings comparing Mr. Teitel’s leadership to that of his predecessor, Jinx Perullo, whom students heralded as an advocate who fought for students, taking on the city’s teachers union in order to hire more talented teachers. The fight left her exhausted, according to an account by a Manhattan Institute scholar and former Stuyvesant parent, Sol Stern, and Ms. Perullo resigned.

At her resignation, a student reportedly gave her a pin labeled “Kids First.”

A return to that kind of cooperative relationship with administrators is the StuyWatch Web site’s goal, its founders said.

A member, Daniel Giansante, of Greenwich Village, said reaching a resolution should not be difficult. “This isn’t the Vietnam War. We’re not protesting civil rights,” he said. Requesting a voice in school policy, he said, should be “just a small thing.”