While her contemporaries "liked" one another's posts on Facebook and Instagram, or nattered on Snapchat and WhatsApp, Joanna was using the Internet to track down a fake passport and find her way to Syria.
In the process, she also found herself in contact with one of the women directly involved in the Paris attacks.
"I was trying to get a passport to leave for Syria. I had names and contacts, and this woman spoke to me on social media; she wanted to go to Syria with someone, she didn't want to go alone.
"She was also trying to control everything I was doing," says the 15-year-old, from the outskirts of Paris.
Like many in France, Joanna (not her real name) was brought up as a Catholic. But in her teens, she decided to convert to Islam.
A naturally bright, curious child, Joanna was a fast learner, according to her mother, Jeanne (not her real name), and deeply interested in religion from a young age.
"As a baby she was the first one to learn how to speak, to learn her colors. She was very active, and got bored very quickly," she remembers. "When she was six or seven she was very curious about religion, and decided she wanted to work in a church."
But as she grew older, Joanna says her religious fervor took her in a different direction.
"I was very into Christianity [but] at some point Christianity didn't bring me what I wanted; it didn't answer some of my questions, and I didn't like the traditions, so I got into Islam instead."
She says her conversion meant that she "became another person."
"I had a lot of Muslim friends, I read the Quran - when I read one page I wanted to read another and another; this religion seemed to coincide with what I wanted."
Islam 'a passion'
And she says her experience of Islam, as a convert, was very different to that of those who are born into the religion.
"People who grow up in a Muslim environment, they don't live it like a passion, but for me, it's something I love ... a passion. I can't live without it."
That obsession meant she was easy prey for those looking to radicalize and recruit Muslims to terror groups like ISIS.
"I received loads of messages from them, I was constantly in touch with them ... They made sense of my life, made me think I had an important role on Earth. I really felt like I was loved, even more than by my own family."
Joanna's growing isolation from her family, who, as non-Muslims, found it hard to understand her decision to convert, meant the recruiters were able to manipulate her.
"They are sneaky because they know exactly how your family is going to react to the situation," she says. "When you are not from a Muslim family, they know that your parents won't accept the hijab, or halal food, they know that [your parents] are going to forbid you from going to the mosque.
"They know all of this and they tell you that your family will reject you, that they will stop loving you; they say that the only ones who love you are your brothers and sisters in Islam. Then, when things deteriorate with your family, you turn to them instead."
At first, her mother wrote off Joanna's obsession with Islam as some sort of teenage rebellion.
"She was avoiding any discussion," Jeanne explains. "A lot of people tell you it's just teenage years, but it's not just that -- it's different with radicalization."
She says she finally realized that something was seriously wrong when Joanna called her an "infidel." That's when she called a national helpline.
"It was awful. I felt really bad, so guilty - as a mother, our first reaction is guilt," she explains. "We try to find the reasons why our child suddenly changed, we think of what we could have done to prevent this from happening.
"What should I have done? I smoke, so I thought maybe I shouldn't have smoked? Maybe I wasn't religious enough? I felt my daughter didn't love me anymore, that she was rejecting me."
Thanks to her mother's intervention, Joanna is now one of the youngest participants in France's program (Centre de Prévention contre les Dérives Sectaires liées à l'islam, translated as Prevention Center against Sectarianism related to Islam) to bring Muslims back from the brink of radicalization.
Her counselor, Laura Bouzar, says it is no easy task: "It's really hard to make them doubt." She says the key is to encourage them to think for themselves rather than accept the militants' propaganda.
"They think they know the truth ... [That] ISIS is good, we are bad. We are here to make them doubt. We are here to make them think for themselves, to ask tough questions. And to stop thinking ISIS is right," she says.
Along with taking part in regular counseling sessions, Joanna must report to the police every day, but the teenager still fears that she will return to her old habits and relationships, which has made her extremely wary.
"I took the decision not to get a new phone; it's better this way. Without a phone and internet, there's no one to tell me what to do anymore, and for now, I don't feel like going back on social media.
"I'm afraid that one day I'll feel lonely and I'll fall into the trap again."
Nowadays she is keen to distinguish between Islam and ISIS; with support and counseling from Bouzar, she has broken away from the latter, but is still devoted to the former.
"I am not talking about Daesh - it's a sect, it has nothing to do with Islam; these days I am learning how to like Islamic traditions and the good things about Islam."
But she says she worries for others like her whose curiosity or vulnerability could lead them down a dangerous path.
"Any person who is not at ease with themselves can be trapped," she says. "They [the recruiters] always know the right words to use; they aren't dumb, they're smart. It's manipulation, and unfortunately I got trapped."
Joanna's message to those, like her, at risk of being targeted by ISIS is to stay alert.
"You should always be careful on the Internet. Don't even go there, don't speak with them, don't take any risk. [And] for those who are already radicalized, please open your eyes to reality. Don't go to Syria: it's suicide, it's death."
A kind of hell
Joanna was lucky enough to get out before she found herself inside ISIS territory; fellow Muslim Hanane was not so fortunate.
Lured to Syria by ISIS propaganda that promised "a paradise" without racism or greed, guided purely by the principles of Islam, instead she found herself in a kind of hell.
When Hanane (not her real name) refused to marry an ISIS fighter, she says she was imprisoned, beaten and accused of being a spy; the friends who had been like "sisters" turned against her.
"I didn't understand," she recalls. "These girls ... said they loved me, they said I was smart and important to them; they invited me to their house, we ate together, we were doing everything together.
"I never did anything wrong to them, but they wanted me dead because I refused to get married."
An ISIS court ruled there were not enough witnesses to convict her, she says, and after several weeks in detention she managed to convince her jailer to let her go, and made a dash for the border.
But her ordeal did not end when she returned home to France; there too, she says, she was considered an enemy.
"When I got back to France I was considered as a girl who tortured people, like a monster who came back pretending to be a victim. I didn't hurt anybody there," Hanane insists. "The only person I hurt was myself."
Now under police observation, she agreed to speak to CNN on condition we did not identify her.
Like Joanna, Hanane is now part of the deradicalization program (CPDSI) run by Dounia Bouzar (Laura Bouzar's mother), but she too is finding it tough to adjust to "normal" life, away from the ISIS recruiters.
"I don't have my group anymore. I don't have my shield," she tells Dounia, admitting she feels vulnerable, and nostalgic for the community she was once part of.
But Dounia Bouzar insists that there is light at the end of the tunnel for those brave enough to break away, to resist the lure of the recruiters: "What doesn't kill you makes you stronger. There is a future."
"I tell parents that their children are going to make their way through this difficult moment: 'Your child will save others.' I'm sure that their experience will help France in the fight against terror," she says.
"We are [part of] a human chain, and we become a wave, crashing against these ISIS words: 'We will win, because we love death more than you love life.' [But] WE will win, because life is stronger than death."