BOSTON (AP) — Sneaking a cigarette in the school bathroom? How quaint. Today's teens have taken to vaping, an alternative to smoking that's so discreet they can do it without even leaving the classroom.

Health and education officials across the country are raising alarms over wide underage use of e-cigarettes and other vaping products.

The devices heat liquid into an inhalable vapor that's sold in sugary flavors like mango and mint — and often with the addictive drug nicotine. They're marketed to smokers as a safer alternative to traditional cigarettes, but officials say they're making their way to teens with surprising ease.

A new wave of smaller vapes has swept through schools in recent months, officials say, replacing bulkier e-cigarettes from the past. It's now common in some schools to find students crowded into bathrooms to vape, or performing vape tricks in class.

"We've seen significant increases across the student body," said Robert Keuther, principal at Marshfield High School on the south shore of Massachusetts. "This is not something specific to one group of kids. It's across all of my grades, nine to 12. It's all students."

Vaping devices are notoriously difficult to detect for schools, often leaving behind only a quick puff of vapor and a light fruity scent. Students get away with it in bathrooms, halls and even classrooms, where some say they exhale the vapor into their shirts.

Although buying e-cigarettes is illegal under age 18 — and some states have bumped the minimum age to 21 — students say they can buy them online or from older friends. Some say there are dozens available for sale in school hallways at any given time.

The rise of teen vaping has sparked concern among parents, politicians and federal health authorities, who on Tuesday announced a nationwide crackdown on underage sales of e-cigarettes.

The Food and Drug Administration issued a warning to retail stores as part of its new operation against illegal sales. It also requested marketing and design documents from the maker of the Juul, a popular vape product that is shaped like a flash drive. The agency says it's looking into whether certain features are specifically appealing to young people.

Some schools have been inundated by the Juul, which dispenses a flavored vapor containing higher concentrations of nicotine than tobacco cigarettes.

The device's maker says it's intended only for adults trying to quit smoking. Its website aims to block underage customers, and the company says it supports legislation to raise the minimum age for vaping products to 21 nationwide.

But critics say the Juul's sweet flavors and stealthy design seem to be aimed at kids. In an April 18 letter to the FDA, a coalition of medical and health groups called for a suspension of online sales until authorities create stronger rules against underage sales.

Similar measures have been backed by school leaders including Keuther, who oversees 1,300 students south of Boston.

"There's a reason why it's marketed that way," he said, referring to the Juul's concealable design. "We wish there was a way to curb that, because the industry is clearly targeting younger kids."

Many schools are pushing back with education campaigns against vaping. Health and gym classes feature new lessons on potential risks. Teachers are being trained on what to look for. Schools are producing online videos on the dangers of e-cigarettes.

At some schools, vaping penalties have been raised to an automatic suspension. Several in New Jersey now require drug tests for offenders, partly because vapes can dispense marijuana.

New York's Plainedge High School was among the first to install new bathroom sensors that can detect e-cigarette vapor and immediately alert administrators. Few students have been caught so far, but officials say that isn't a sign of failure.

"The truth of the matter is the kids see it, they know what it is — and it in itself is a deterrent," said Edward Salina, superintendent of Plainedge Public Schools.

Whether schools' efforts are making a broader difference has yet to be seen, though. Teen vaping decreased for the first time in 2016 after rapidly rising for years, according to the Centers for Disease Control and Prevention. Updated numbers are expected in June.

Medical experts are still trying to understand the potential risks tied to vaping. Most agree that it's safer than smoking traditional cigarettes, but little is known about long-term effects.

And while research has found trace amounts of chemicals like formaldehyde in many e-cigarettes, it's unclear whether they exist at levels that can cause health problems.

Some students say that it's better to be vaping than using cigarettes or other drugs. Cameron Uldricks says he vapes almost every day but has never smoked tobacco. And even though it landed him a suspension from his high school near Columbus, Ohio, the 17-year-old said he has no plans to cut back or quit.

"Honestly they treat it like frigging cocaine," said Uldricks, a sophomore at Worthington Kilbourne High School. "What would they rather have me do, smoke cigarettes or vape?"

While high schools have typically faced the widest problems, some districts say it's now spreading to middle schools.

At Chickamauga City Schools in rural Georgia, officials instituted a three-day suspension for vaping this year after catching several students doing it at the middle school. Superintendent Melody Day said it's still unclear whether the crackdown has worked.

North of New York City, officials at South Orangetown Middle School hosted a forum for parents last month after starting to see some cases. Students caught vaping go through counseling on risky behaviors, which officials hope will halt any escalation to further drug use down the line.

"They'll do that after taking the risk here," said Karen Tesik, principal at the middle school. "This is the age group that is the most at risk for taking that first step."

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Binkley can be reached on Twitter at @cbinkley