HARTFORD, Conn. — Passing notes in study hall or getting your best friend to ask a boy if he likes you or, you know, LIKES you, is so last century. Nowadays, teens are snapping naked pictures of themselves on their cell phones and sending them to their boyfriends and girlfriends.

Many of these pictures are falling into the wrong hands — or worse, everyone's hands, via the Internet — and leading to criminal charges.

Some parents are aghast.

"I just don't understand why kids would do a stupid thing like that," said Rochelle Hoins of Castle Rock, Colo., where 18 students in her twin sons' middle school sent around nude pictures of themselves last year. "We did dumb things when we were kids, but not like that," said Hoins, whose sons were not involved.

Similar cases have been reported in New Jersey, New York, Alabama, Utah, Pennsylvania, Texas and Connecticut.

"It used to be that kids would make mistakes, and it was local and singular and everyone knew it was part of growing up," said Catherine Davis, a PTA co-president in Westport, Conn., who had a frank talk with her two sons after several students' nude self-portraits recently spread through her community. "Now a stupid adolescent mistake can take on major implications and go on their record for the rest of their lives."

School administrators in Santa Fe, Texas, confiscated dozens of cell phones from students in May after nude photos of two junior high girls began circulating. The girls had sent the photos to their boyfriends, who forwarded them to others, officials said.

In La Crosse, Wis., a 17-year-old boy recently was charged with child pornography, sexual exploitation of a child and defamation for allegedly posting nude photos of his 16-year-old ex-girlfriend on his MySpace page. The girl had taken the pictures with her cell phone at her mother's home and e-mailed them to the boyfriend, authorities said.

"They were pretty graphic," said sheriff's Sgt. Mark Yehle. "I think they just do it to impress their boyfriends. When he breaks up, he 'vents,' in his words, by posting them. He apparently didn't think there was anything wrong with it. He didn't know it was illegal."

Psychologists said the phenomenon reflects typical teenage hormones and lack of judgment, with technology multiplying the potential for mischief. It also may reflect a teenage penchant for exhibitionism, as demonstrated on MySpace and countless other Web sites and blogs.

Brianna Moran, 15, who attends the same school as the girl in the La Crosse case, said she is not surprised by such behavior.

"If you look at people's MySpace, all the pictures are slutty," she said.

In suburban Syracuse, N.Y., several teenage girls sent naked pictures on their phones to their boyfriends, only to learn that another boy had collected them from the Web and was trying to sell a DVD of them.

Some boys are photographing themselves, too. In Utah, a 16-year-old boy was charged with a felony for sending nude photos of himself over a cell phone to several girls. Four middle school students — two boys and two girls — in Daphne, Ala., took photos of themselves on their cell phones and traded the images back and forth, authorities said.

Some nude photos have even turned up in parents' e-mail inboxes.

The images are complicating the work of investigators whose job is to find exploited children. Authorities trying to identify youngsters in naked photos are increasingly discovering that the teens themselves took the shots, said John Shehan, a director at the National Center for Missing and Exploited Children.

Connecticut police Sgt. Jim Smith, who investigates cybercrime and online child pornography, conducts seminars in which he warns parents about the use of cell phones to send nude pictures.

"It's often so spur of the moment that they're not thinking about where those images might end up," Smith said. "They might think it's just fun and games ... but these images can really spread like wildfire."