Video taken by a stalker shows a teenage girl, completely aware, being followed.
Information posted online revealed where she worked - and when, CBS News correspondent Seth Doane reports.
Later, she was tailed home.
But all of those details came from the girl herself: Adrienne Chavez posted them on her own Web page.
"You don't think it's a big deal because everybody has personal information online, so you know, I never thought twice about it," Chavez said.
Her stalkers - fellow classmates at Thomas S. Wootton High School in Maryland, participating in a project to show how vulnerable teenagers make themselves on the Internet.
"She caught on at one point that people were following her, but if it was a really good stalker she might not have caught on before it was too late," said classmate Amy Ding. "And she would have never known until something serious happened."
According to one recent survey, 61 percent of teens reveal where they live on their profiles, 29 percent give their email addresses and 29 percent even post their last names.
All of which is a bad idea, says Wyoming investigator Flint Waters, who's made it a personal crusade to track Internet predators.
"We find where the child has been online, or the teen has been online and the single picture of themselves wearing the school jersey, or wearing some other distinguishing clothing lets this predator figure out where they are and how to reach them," Waters said.
In fact, in a quick survey of social-networking sites, we found one girl posting her cell phone number online - another one names her college, and even the dorm she lives in … including the number on her door. One young man shows his phone number and his employer.
Internet safety expert Parry Aftab, who spoke to the school, says teens continue to let their guard down online, when they should do the opposite.
"The kids have a disconnect, a total disconnect between reality and virtuality. When they are online they think it's a special world that they can control - what they don't realize is that it's special world, alright, but they have no control over it at all.
The project was so realistic it hit a nerve.
"Kids got mad at us for saying, 'you know, you're not our parents, you can't tell us what to do,'" said student Ben Gordon. "And we said, well, we're not. What we are saying is 'don't be stupid about it', don't show the world what you are doing in your spare time,' because it will get you in trouble."
How to stop it?
Aftab says: "We can talk to parents, but get in line behind 'don't do drugs, eat healthy, exercise' - it just doesn't get there. And parents are absolutely convinced that their child would never agree to a meeting off line."
But Aftab says there are some things teens can do.
In the end, as the students at Wooten learned. It is up to the teens themselves to be smart with their online behavior.
"What we are saying is 'don't be stupid about it, don't show the world what you are doing in your spare time' because it will get you in trouble."