When a drug-sniffing Belgian Malinois found two grams of cocaine inside a student’s backpack at Sachem High School North early this year, the entire student body knew about it immediately. The cocaine was synthetic and the student was a volunteer for a presentation being given inside a packed gymnasium.Dogs in hallways, their noses turned to the air for the scent of contraband, are increasingly common in public schools on Long Island and in Westchester, New Jersey and Connecticut, school and law enforcement officials said. While many said that the dogs help keep drugs away, some students and civil rights groups called the dog searches not only inaccurate but also frightening. The atmosphere created by taking police dogs into schools is “incompatible with nurturing environments that are supposed to be conducive to adolescent education,” said Donna Lieberman, executive director of the New York Civil Liberties Union.

The best-trained dogs have an accuracy rate of 85 to 90 percent, said James Greco, head trainer for Long Island K-9 Service, which contracts to conduct drug searches in 15 Long Island public schools, as well as three private schools in Westchester and another in Hartford.

“No dog is 100 percent accurate,” Mr. Greco said.

At Sachem High School North on Long Island, dog searches were instigated as a “preventative deterrent,” said Charles J. Murphy, the district superintendent. Dogs have searched the school twice since January, and no narcotics have been found, though the dogs did indicate several lockers, which were found to contain nothing illegal, Dr. Murphy said. Since the first dog visit, no student has been found with drugs on school property; previously, about one student was caught with marijuana every month at Sachem North, he said. There are no statistics on how many schools nationwide use dogs to search for drugs, said Brittany A. Brown, a spokeswoman for the Drug Enforcement Administration. Policies on the use of dogs in schools vary by school districts around the region.

Of the districts that allow dog searches, some require students to remain in classrooms when dogs are present; others allow dogs and students to be in the same areas. None allow dogs to search students, only lockers, parking lots and other common areas, school officials said. The searches are never on a set schedule, and their date is not revealed in advance to the students, officials said. In North Caldwell, N.J., dogs have searched West Essex Regional High School twice this year, said Joseph F. Clark Jr., chief of the North Caldwell Police Department. Only one locker had to be searched, he said, and it was deemed clean.“The point isn’t so much to make arrests as it is to just make it as hard as possible for anyone to bring drugs into school,” Chief Clark said.

The Connecticut Association of Boards of Education advocates using drug dogs in schools, said Vincent A. Mustaro, the group’s senior staff associate for policy services. Joseph V. Erardi Jr., superintendent for the Southington School District in Connecticut, said the district works with the local police, who search the high school randomly and at no charge. He said that after numerous searches during the last five years, the dogs have not led authorities to any illegal narcotics. Dr. Erardi said he did not believe that the searches interfered with the education process. “It is my belief that when dogs are on campus, teaching is not disrupted,” he said.

The issue made headlines in Connecticut last year when dogs searched Canton High School the day before the senior prom in June. Fifteen students whose lockers or cars were indicated by the dogs were pulled out of class. Drugs — a small amount of marijuana — were found only in one car, and a student was arrested. Kelsey Jones, 18, now a freshman at the North Carolina School of the Arts, was one of the Canton students whose locker was searched because of a dog’s reaction. She recalled being escorted from class and forced to stand by her locker as an armed officer searched her belongings.

“I was thinking, ‘Oh my God, what have I done?’ ” she said. “I felt extremely accused.”

Jonathan Matthews, legal director of the American Civil Liberties Union of Connecticut, criticized the school board for the search and succeeded in getting a small policy change. Now when a search dog indicates a locker may hold drugs, a school administrator must take note and then return to search the locker when the police officers are gone. “It made the police presence a little less than it was before, “ Mr. Matthews said.

The demand for drug-detecting dogs in New Jersey spurred Debra Stone to start Sniff Dogs, a company based in Summit, in September to provide dogs to private homes to search for drugs. Since then, she has contracted with schools from Bergen County to South Jersey to conduct searches. “Every month the number of schools increases,” Ms. Stone said. At Mount Vernon High School in Westchester, law enforcement authorities with dogs search for drugs when they find it necessary, said Desiree Grand, a district spokeswoman. She said there had been one search with dogs in the past year.

Karen Fisher, 14, a freshman at the school, said she thought the police dogs were intimidating and ineffective at rooting out drugs. She said that since the search, she had seen marijuana inside her school three times.

“It’s not necessary to bring dogs here,” she said. “They shouldn’t do it.”