Crime is down at city schools, but frustration may be about to go way up.

When students return to class after spring break, they may have to pass through the mobile airport-style metal detectors police will be setting up at different schools each day. Police will be looking for students attempting to carry guns, knives, box cutters, and other weapons to class.

“The days of anything goes are over,” Mayor Bloomberg said yesterday at Abraham Lincoln High School in Brooklyn, where he announced that police will start making the unannounced visits at middle schools and high schools at the end of the month.

While police will focus on the most troubled schools, Mr. Bloomberg said metal detectors could pop up at any school on any day.

The leaders of the teachers and principals unions say the hassle of the scanners is worth it if they help keep students and educators safe, but others worry that it could be sending the wrong message.

“I don’t understand. Are we running prisons or are we running schools?” the chairman of the City Council’s Committee on Education, Robert Jackson, said.

The city already has installed permanent scanners in 82 of the city’s most violent schools. About 100,000 students pass through them each school day. So far this year, the schools have confiscated 307 weapons, including 20 guns, and 1,355 “dangerous instruments.”

When students return to school after spring break, they will find new signs posted on the walls warning that they can be searched at any time. The random searches will kick off April 24.

“If children are late and miss the entire first period, what happens? How are they going to deal with that?” Mr. Jackson said.

In September, about 1,500 angry high school students walked out of DeWitt Clinton High School in the Bronx to protest new metal detectors.

Students have complained about having to stand in long lines just to get through the front door of schools with scanners.

Under the new plan, police will pack up their metal detectors and move on after just a day or two.

The principal of Abraham Lincoln High School, Ari Hoogenboom, said yesterday that it took the school about two weeks to adjust to having metal detectors.

“Our students don’t wait any more than three to five minutes every morning. Once you learn a system, once you apply it fairly to all students, things can work quite well,” Mr. Hoogenboom said. The school started using metal detectors in September and has since moved off the city’s list of most violent schools.

The number of violent crimes in city schools has dropped 11% since last year, but criminal mischief increased by 56%, largely due to a crackdown on graffiti, city officials said. The city hired an additional 200 school safety officers in September, bringing the total to more than 4,600.

Other school districts around the country have turned to similar random safety checks, according to the executive director of the National School Safety Center, Ronald Stephens.

“There’s only about six hours in the education day, so you have to ask at the end of day, do you want to have metal detector searches every single day to address the crime problem, or would it be more effective to have random searches,” Mr. Stephens said.

Mr. Bloomberg also announced yesterday that the number of the city’s “impact schools” would drop to nine at the end of the month from an all-time high of 16.

To crack down on soaring violence, Mr. Bloomberg started the “impact school” initiative two years ago. He flooded the city’s most crime-plagued schools with additional teams of police officers and stepped up safety measures.

Violent crime at the city’s current list of 11 impact schools dropped by 43% since last year, compared with 11% citywide.

Four schools will move off that list at the end of the month including Abraham Lincoln and another two high schools – John F. Kennedy in the Bronx and Newton in Queens – will be added.

The city’s schools chancellor, Joel Klein, said yesterday that crime continued to drop at schools even once they moved off the impact list. He said a “fundamental change in the culture” at schools had helped to eliminate violence.

“Random screening across the city is a very important first step,” the president of the United Federation of Teachers, Randi Weingarten, said. “It’s also essential to make sure that codes of conduct are genuinely enforced so that a few disruptive kids aren’t able to prevent all the other students from getting the best education possible.”

The executive director of the New York Civil Liberties Union told the Associated Press that her organization plans to examine the legality of the surprise scanning. “This moves us yet another step closer to a surveillance society,” she said. “First, we have unannounced searches in the subway. Now in the schools. It’s a short step to unannounced searches in the streets.”