Ever since Justin, a 15-year-old high school freshman, tried marijuana on his birthday two years ago, he has smoked almost every day, several times a day, he said.

“If I smoke a blunt, after that blunt I’m going to be chill,” he said on a recent morning at a corner deli near his school, the Bronx Design and Construction Academy. “I’m not going to be stressing about nothing at all.”

Another boy came by and flashed two glass tubes of smokable flower. More students were smoking across the street in a doorway and on a stoop. On another corner, a smoke shop frequented by children in backpacks and uniforms opened about half an hour before the first bell.

While it has long been common for some teens to smoke marijuana, teachers and students say that more and younger students are smoking throughout the day and at school.

There is little definitive data on marijuana use among children, and what information is available can sometimes offer a contradictory picture. Disciplinary data from the city education department reflects a 10 percent increase in alcohol- and drug-related offenses this year compared to 2019. But a city survey found teen cannabis use had declined in 2021, the same year that the state legalized marijuana for recreational use, to the lowest level recorded since the question was added to the survey in 1997.

Still, two dozen students and teachers at public, private and charter schools across the city said in interviews that some classrooms were in disarray as more pupils showed up late and high.

They said that with the proliferation of unlicensed smoke shops and the availability of vape pens and edible products, cannabis has never been more accessible and inconspicuous. They relayed accounts of students taking hits of vaping pens when teachers turned their backs, of bathrooms and stairwells becoming smoking lounges and of the smell of weed wafting through school hallways.

Teachers across high schools in the city said it was rare to catch students in the act of smoking, given the increasing ease, leaving reports to be made based on more opaque judgment calls of the students’ smell and behavior.

“It really feels like this unstoppable tide that we’re futilely trying to suppress,” said America Billy, 44, who has been teaching at a public high school in Manhattan Beach, Brooklyn, for over a decade. She said it was hard to know whether a student was out of it because of a lack of sleep, family stress or drugs.

In December, a former principal, April McKoy, described in a letter how students’ cannabis use had spiraled out of control during her last two years in charge of City Polytechnic High School of Engineering, Architecture, and Technology in Brooklyn.

“It felt like more and more were using without knowing the source, impact or consequences of early marijuana use,” Ms. McKoy said in the letter, adding that students had returned after the pandemic “sad, isolated and trying to find ways to cope.”

Freshmen were selling cannabis to each other, and she said she witnessed a smoke shop sell edibles to 14-year-olds with police officers nearby. On another occasion, she sent four students to the hospital because they were sickened from contaminated edibles, she said.

The proliferation of unlicensed smoke shops, which the city says may number as many as 1,500, could be one factor driving marijuana use among children, officials said.

Gale Brewer, a city councilwoman, said that though she had counted fewer than 10 of them in her district on the Upper West Side of Manhattan in September, there were 64 by March. Several school administrators have complained to her about merchants selling joints and infused candies as well as high-potency concentrates and vapes to students.

“We were all saying we need social workers, we need psychologists, we need mental health support in the schools,” she said. But dealing with smoke shops selling to children “was not on the list.”

Mayor Eric Adams has vowed to crack down on unlicensed smoke shops, though he has not taken sweeping action. In February, his administration filed nuisance abatement lawsuits targeting a handful of stores where the police said underage auxiliary officers were able to buy marijuana. At the same time, Alvin Bragg, the Manhattan district attorney, sent letters to shops threatening to evict them, but so far his office has not initiated any proceedings.

In Albany, state lawmakers passed budget legislation in April that expanded the powers of state cannabis regulators and tax authorities to close unlicensed stores and impose hefty fines for illicit sales. Mr. Adams’s office praised the measure, but urged the state to give the city additional enforcement powers to rein in illicit smoke shops.

Jenna Lyle, a spokeswoman for the Education Department, said schools offer a range of programs aimed at addressing and preventing substance abuse among students, including specialists who provide counseling in schools. But last year, there were just 280 specialists for the city’s 1,600 schools, Chalkbeat has reported.

Esther Lelievre, a cannabis activist who conducts educational workshops at schools at community centers, said that many of the students who use cannabis said they had started out vaping nicotine, a phenomenon that was on the rise before the pandemic. Few of the students she has worked with obtained their marijuana from smoke shops, she said. Most got it from friends who had access to a dealer or to cannabis at home.

At the Bronx Documentary Center, a nonprofit photo gallery near Justin’s school, students in its journalism program have set out to bring more awareness to cannabis use among kids after witnessing the change in their peers.

They mapped all of the smoke shops and schools in the neighborhood with push pins, and connected those that were closest with rubber bands. Showing the map during a recent evening class, Cara-Star Tyner, 15, noted that one of the rubber bands did not stretch.

“That’s how close it is,” she said.

One of the shops, Puff Puff Pass 1, was visible through the window of their workroom. On a recent morning, The Times observed two teenagers in backpacks and uniforms make a purchase in the store, then later enter a high school building. Two days later, a man who identified himself as the shop’s owner, Mike Alramada, 35, said he did not sell tobacco or marijuana to students. As he spoke, he was interrupted by teenagers ringing his doorbell to be let inside the shop, which also stocked some drinks and other grocery items.

The journalism students said they were disappointed in the adults who ran their schools, their city and the smoke shops, and they hoped that bringing attention to the issue would finally prompt the authorities to act.

“I hope that adults realize they’re not doing their job,” Alexa Pacheco, who attends a Catholic school in the Bronx, said. “A teenager should not be worried about their friends using drugs.”

Lauren McCarthy contributed reporting.