“I’m afraid that we’re going to be hooking a new generation of kids on nicotine, with potentially unknown risks,” said Dr. Mark L. Rubinstein, the lead author of the study and a professor of pediatrics at the University of California, San Francisco. “With cigarettes, we’ve been studying them for many years, we have a pretty good idea of what the risks are. We just don’t know what the risks of inhaling all these flavorings and dyes are, and what we do know is already pretty scary.”

The industry points to a 2016 British study that says that vaping does not lead nonsmokers to become smokers. But the 2016 Monitoring the Future study, sponsored by the federal government’s National Institute on Drug Abuse, followed students who in 12th grade had never smoked a cigarette and found that a year later, those who used e-cigarettes were about four times as likely to have smoked a cigarette. A study released in January by the National Academies of Sciences, Engineering and Medicine similarly concluded that vaping led students to smoke cigarettes, although it did not determine whether they became habitual smokers or just experimented.

Schools and local officials have stiffened penalties for students caught with vaping devices, suspending and even expelling them, and sent home letters pleading with parents to be on the lookout for a waft of fruit smell and, as one superintendent wrote, “‘pens’ that aren’t pens.”

Several school districts in New Jersey have recently adopted policies requiring any student caught with an e-cigarette to be drug tested, because the devices can be used to smoke marijuana.

Oak Ridge High School in Placerville, Calif., shut down all but two bathrooms during classes in November and placed monitors at the doors during lunch to make sure not too many students are in the bathroom together. (Juul devices, for example, contain as much nicotine as a pack of cigarettes, so they are easy to share.) New Trier High School in Chicago’s northern suburbs is considering installing vaping detectors in bathrooms.

With so many students caught multiple times, some schools have moved from punishment to intervention, requiring students caught vaping to receive counseling or substance abuse treatment.

“Despite all of the boundaries set by families and parents and the schools, and at risk of even expulsion, students are continuing to use,” said Liz Blackwell, a school nurse in the Boulder Valley School District in Colorado. “They don’t want to be kicked out of school, they don’t want to suffer any punishment or discipline, and they don’t want to have a bad relationship with their parents. They continue to use because it’s an addiction.”