Some are wearing them in jest. Others sport them sincerely.

But whatever their motivation, teenagers across the country have been going wild for shirts that bear a chaste declaration: “Virginity Rocks.”

The clothing items became popular online thanks to a social media influencer and are now stocked by a major retail chain with a presence in malls across the United States. The trend has puzzled some school administrators, who have banned the shirts only to face criticism, and other adults, who have wondered if youth abstinence is on the rise.

It can be traced to Danny Duncan, a 27-year-old YouTube personality and prankster, who said he started wearing “Virginity Rocks” shirts in his videos as something of a joke in 2017. Mr. Duncan said that his use of the phrase was “tongue in cheek,” and that most people wore it humorously. But, he added, he is proud to have seen it catch on with young people who champion abstinence.

“I have sex, obviously, but I want people to do whatever they want to do and not be pressured into anything,” Mr. Duncan said. “I sell ‘Practice Safe Sex,’ too, which could be funny but is also a positive message at the end of the day.”

Mr. Duncan has built a following of roughly 3.5 million subscribers on YouTube with his prank videos. In his most popular post, he repeatedly trips and falls while holding a box of pennies in places like a mall escalator, startling the people around him. Other videos show him precariously riding a hoverboard down stairs, jumping out of a golf cart before crashing it into a tree and pulling high jinks while grocery shopping, such as sneaking behind the milk displays. Mr. Duncan said that according to his YouTube statistics, 92 percent of his audience is male, the majority 24 and younger.

And that audience has wholeheartedly embraced “Virginity Rocks.” Mr. Duncan has done branded tours to meet his fans (and sell merchandise). He shared Shopify data that showed millions of dollars in online sales. And a partnership started last year with Zumiez, the teen retailer with more than 700 locations in the United States and abroad, has increased his visibility.

The chain, which specializes in action sports brands, prominently showcases “Virginity Rocks” apparel featuring Mr. Duncan’s name online and in stores, where cardboard cutouts of the floppy-haired YouTuber grin at customers in some locations. The wares, which include bucket hats ($40), lanyards ($12) and slide sandals ($40), alongside the ubiquitous hoodies ($55), have been a hit. Mr. Duncan said Zumiez had told him that it has sold more than 400,000 pieces of merchandise.

Zumiez even invited Mr. Duncan to its annual gathering of top sellers last month in Keystone, Colo., where he presented an award to one of the chain’s top salespeople. (The prize was a Tesla with horns, a reference to a joke from one of Mr. Duncan’s YouTube videos.) He was bowled over by the appreciation from the chain’s associates at the event, he said.

“These employees would be like: ‘Thank you so much. You made me so much money because the shirts are so easy to sell,’” Mr. Duncan said, adding that the merchandise would soon be available in stores in Canada and Australia. “I’ve only been there seven months. It’s kind of mind-blowing.”

Zumiez, which is based in Lynnwood, Wash., said it was company policy not to talk about specific brands.

“We are not going to release sales numbers or disclose our top sellers for competitive reasons, but we do really appreciate Danny’s partnership and the partnership of all our brands,” Chris Work, the chief financial officer, said in an email. He said that there was “truth to the Tesla” anecdote and that he appreciated the help of all the chain’s brands at the annual event.

The success of “Virginity Rocks” merchandise has startled some in Mr. Duncan’s orbit.

“We’re doing these meet and greets for Zumiez and thousands of kids show up and the whole mall is covered in ‘Virginity Rocks,’” Stefan Toler, Mr. Duncan’s manager, said in an interview. “It started as more of a joke, but now it’s an actual brand where we’re outselling Thrasher, Nike, Adidas and all these brands in Zumiez, and we’re like, ‘What the hell?’ Even Zumiez is like, ‘What’s happening?’”

He theorized that those buying the “Virginity Rocks” apparel were split between teenagers who endorsed the message and Mr. Duncan’s fans, who were wearing it ironically. Still, he said, he believes that the popularity of the shirt is making virginity cool among Mr. Duncan’s supporters.

“I’m 32, so back when I was in high school you would not say that, but he’s made it cool with his fans in general,” Mr. Toler said. “If Danny’s fans are virgins, they’re psyched to be virgins.”

Mr. Duncan has a trademark for use of the phrase on clothing, greetings cards and condoms, according to public records. Mr. Toler said policing the trademark was “insane.” Still, he has been able to remove imitators on Amazon by using the site’s brand registry and has been successful when he has sent emails to sites like Teespring, which allows people to customize apparel, asking them remove items, he said.

For some school administrators, the clothing has presented a confounding problem, and they have struggled to address its popularity. Teenagers have been suspended for wearing shirts with the phrase at schools in Oregon, Wisconsin and Missouri. That has outraged parents, who believe the clothes are bearing a positive message.

One student at Roseburg High School, in Roseburg, Ore., was sent home in 2018 after wearing a “Virginity Rocks” shirt. His grandmother wrote a Facebook post wondering why the principal found it offensive. The school, which declined to comment when contacted by The New York Times, told a local news outlet at the time that the shirt would have been disruptive in class, and that the same treatment would have extended to shirts that said “Sex Rocks” or “Smoke More Pot.” The school also gave the student the option to turn the shirt inside out or change into one it provided.

Mr. Duncan, who lives in Los Angeles, subsequently visited Roseburg, filming a video that he posted on YouTube. The video shows him emerging from a sleek black van emblazoned with “Virginity Rocks” and being greeted by a large group of screaming teenagers in a parking lot. He then distributes free “Virginity Rocks” shirts to the ebullient crowd, which begins chanting the phrase with him.

A high school student in the Chetek-Weyerhaeuser school district in Wisconsin and a middle school student in Wentzville, Mo., have also been disciplined for wearing the shirts, according to local news reports. Some of the incidents have been picked up by conservative news sites, which have framed them around the issue of schools banning pro-abstinence clothing in an affront to Christian values.

“I don’t understand why you would ban it,” Mr. Duncan said. Plus, he added, “a lot of kids kind of like it when they’re not supposed to have it.”

Mr. Toler said they might introduce a new set of merchandise to address the issue. “We’ll put a censored bar over ‘virginity,’ but kids will know what it is, so that way schools won’t take the shirts and suspend them,” he said.

On Instagram, the hashtag #virginityrocks leads to photos of Mr. Duncan, selfies of young people wearing clothes with the phrase and memes that are often inappropriate. It also leads to pictures from Zumiez stores.

Mr. Duncan believes that the success of the term is tied to his persona. It would mean something different if it had been popularized by Bam Margera, the professional skateboarder and “Jackass” star, he said, and send yet another message if worn by the pastor Joel Osteen.

“The shirt wouldn’t sell if I didn’t have my face behind it,” he said. “I’m just trying to get it as big as possible.”