NEW YORK - As students approach John F. Kennedy High School in the Bronx, they take off their watches, unbuckle their belts and empty the change from their pockets.

Couples embrace before parting to go to single-sex entrances at opposite ends of the building. There, they place their bags under scanners and walk through metal detectors. If they set those off, they are patted down.

This process, similar to passenger screening at airports, is a daily ritual for the more than 4,000 students at Kennedy, where students are often late for first-period classes after standing in line for 30 minutes or more.

"We know the process," said Hanna Reyes, a junior. "We know what to take off, what shoes will set off the metal detectors. But you do feel like a prisoner."

This morning routine is not unusual in New York City, where 65 high schools have similar setups.

Metal detectors were first used in New York City schools in the 1980s, said Benjamin B. Tucker, who heads the Office of School Planning and Safety in the Department of Education. "This is a tool to keep schools safe," he said. "It has always been seen and used that way."

In the Los Angeles Unified School District, school security guards are required to conduct random searches in at least one classroom every day in middle and high schools. Philadelphia put large metal detectors in all city high schools five years ago. Most of the schools have two metal detectors, and students there typically arrive an hour before classes.

In most cases, New York City school officials request metal detectors from the city's Education Department. Most of the high schools in the city now use metal detectors, and so do a few middle schools, Tucker said.

But at Kennedy, where the metal detectors were installed this fall after a student was stabbed to death outside the school in the summer, students and teachers are unsure that the new security measures have made the school safer.

"The problem wasn't really weapons, it's people fighting and things like that," said Leidy Colon, 17, a Kennedy senior.

To get into Kennedy in time for her 8:20 a.m. class, Hanna Reyes leaves her Washington Heights home by 6:30 a.m. "I don't want to take any chances, I really don't," she said. "It's frustrating to us. You kind of dread going to school."