Social networking means many teenagers are never far from a bully, harassment or unwanted sexual messages.

Teens are joining social-networking sites at younger ages, spreading bullying and other ugly behaviors to the virtual realm. As a result, officials have been ramping up efforts to prevent threats like online harassment and “sexting.”

“In the past, you dealt with a bully on the playground. You left and it was over,” said Officer Marc MacDonald, a school resource officer with the Fairfax County police. “These kids are 24 hours a day into social media, on their phones, everywhere they go. They can’t just walk away from it.”

One in three teens ages 12 to 17 have been subjected to online harassment, according to a 2010 Pew Internet and American Life Project presentation. Fifteen percent said they received sexting messages.

Locally, Lakelands Park Middle School in Gaithersburg notified parents last year after authorities found students making threats online. A 14-year-old girl in Prince William County was charged with stalking for posing as a boy on Facebook to strike up a relationship with another girl. And Montgomery County officials busted a middle school boy who rented out his iPod Touch so others could view photos of nude female classmates.

Such cases have spurred authorities to put cyberbullying and other social-media-related crimes on their radars.

President Obama held an anti-bullying conference last week, and the D.C. Bar Association’s upcoming youth law fair will focus on cyberbullying.

The law fair aims to teach teens that cyberbullying can lead to anxiety, depression and poor performance in school, just like physical bullying, said Vanessa Taylor, the association’s events and outreach coordinator.

“Once you send a message, you can’t take it back,” she said.

The problem is especially prevalent among middle schoolers, experts said. That’s when youth usually begin going online without assistance from their parents and start using social media sites like Facebook, Twitter and YouTube, said Michelle Boykins, spokeswoman for the National Crime Prevention Council.

Boykins said online aggression often starts after an in-person dispute.

“Something happens at school or a mall that’s a confrontation that turns into a war of words or harassment online,” she said.

And sometimes, those physical confrontations themselves end up the Web. Last week, a video of a fight at High Point High School briefly surfaced on YouTube. Students have said the fight was posted to bring attention to violence at the school.

“This was a clear and explicit cry for help,” said Prince George’s school board member Edward Burroughs. “In other cases, there are times when we have students post fight videos just for entertainment or for no good cause.”

Police and school officials say they sometimes are hamstrung in efforts to discipline offenders. Forty-four states and the District have bullying laws, but only six include language specifically about cyberbullying, according to the Cyberbullying Research Center. Legislators in Maryland and Virginia have pushed to explicitly include text messages and social networks in harassment legislation.

Social networks, though, are also stepping up safety measures. Formspring, a social network with a reputation as a forum for bullying, announced that it will work with the Massachusetts Institute of Technology to better detect online bullying. And Facebook is preparing to debut a system to let users report abusive content to someone they know — like a parent or teacher — in addition to asking the site to remove it.