Then she went off to pursue violent jihad.
She has turned up often on social media, exulting the terrorist group's ideology and calling for attacks against the West. She has posted photos of AK-47s and of executions carried out by ISIS fighters.
On what appears to be her Tumblr blog, posts made after Aqsa left Scotland advocate becoming an ISIS bride, summoning the strength to walk away from the family one is born to and join a new family of jihadists. Some of the posts read like the writings of a zealot hard bent on indoctrinating others. Some posts are total teenager.
One Tumblr entry mocks the notion that authorities would confiscate passports of those who tried to join ISIS.
"Wow wallahiil Adheeem (I swear to God) biggest joke of this week," it says.
Stunned and horrified, Aqsa's family issued a public statement last weekend.
"You are a disgrace to your family and the people of Scotland," it said. "Your actions are a perverted and evil distortion of Islam."
"You are killing your family every day with your actions," the family warned. "(We) are begging you stop if you ever loved (us)."
But their daughter isn't the only one who should shoulder the blame for her possibly joining the terror group, her parents said, blasting investigators who they said have monitored Aqsa Mahmood's social media accounts for months.
The teen's family wants to know whether authorities could have done more to stop the three girls from leaving after at least one of the teens allegedly communicated with Aqsa online.
Promises made online
Though it's not entirely clear what sparked Aqsa's interest in ISIS, she watched sermons and met people online who persuaded her to join ISIS, authorities told CNN.
ISIS is more aggressively recruiting women
than any other terror group has and is luring them by painting a false narrative about what life is like in Syria, said Michael Steinbach, the head of the FBI's Counterterrorism Division.
"We have seen everything from a female fighter -- dedicated groups of women fighters -- and those who have come over to support foreign fighters by marrying them," he told CNN in an exclusive interview earlier this month.
ISIS is targeting young people with a fervor.
In the United States, the FBI knows of children as young as 15 whom ISIS has tried to recruit, Steinbach said.
He is very worried about how easy it is, once someone is in Turkey, to make it into Syria. And it's extremely difficult to track every American who might travel abroad to join terrorist groups like ISIS, he said.
U.S. law enforcement and intelligence agencies don't track individuals leaving the United States to vacation in Europe.
"Once you get to Europe, you can easily get down to Turkey and into Syria," Steinbach told CNN.
Last week, an airport surveillance photograph showed three young women
, dressed stylishly, walking through a London terminal.
They were 15-year-old Shamima Begum, 16-year-old Kadiza Sultana and 15-year-old Amira Abase.
Shamima's family said she had voiced a desire to help suffering Syrians. Amira told her father she was going to a wedding. But instead the three hopped a flight to Istanbul.
Their families don't know where they are and have pleaded for them to return, telling them not to worry, that they aren't in trouble, but just to come home.
On CNN on Tuesday, attorney Aamer Anwar said the Mahmoods believe their daughter got in touch with the three British girls on Twitter. He said he knows that Shamima and Aqsa communicated via direct message, but the family doesn't know what the girls wrote.
On Tuesday, London Metropolitan Police said investigators believe the teens have crossed into Syria.
Anwar said that Aqsa's family thinks that authorities have been closely monitoring Aqsa's social media activity and that investigators didn't tell the families of the teenagers that she had had contact with the three British teenagers.
"One would expect a courtesy of a knock on the door of those families to advise them that their children may be on the cusp of radicalization or be going off to Syria," he said. "Yet that never happened."
CNN asked Metropolitan Police about the lawyer's statements and a spokesperson declined to comment, citing an ongoing investigation.
While the families wait and worry, a question looms: How could bright girls from solid families consider abandoning their lives to join a terror organization known for horror-show brutality?
Diary of an ISIS bride
The Tumblr blog
that Aqsa's family and their lawyer believe belongs to the teen explains to potential recruits why they should join ISIS and how, in practical terms, to travel to join the militants. The account, which appears to date back as far as the spring of 2013, before she left Scotland, is full of images. There are pictures mocking Syrian President Bashar al-Assad, photos of injured children, various Islamic fighters and the hacker collective Anonymous' iconic Guy Fawkes mask.
Often, after each entry, dozens of people "like" what's written. Some reblog her entries.
It's a misconception, she writes, that only the poor or marginalized are drawn to ISIS. A September 11, 2014, post titled "Diary of a Muhajirah"
reads, "The media at first used to claim that the ones running away to join the Jihad as being unsuccessful, didn't have a future and from broke down families etc. But that is far from the truth."
"Most sisters I have come across have been in university studying courses with many promising paths, with big, happy families and friends and everything," it continues. "If we had stayed behind, we could have been blessed with it all from a relaxing and comfortable life and lots of money."
That seems to reflect Aqsa's upbringing.
In the 1970s, her father moved to Glasgow from Pakistan and became the first Pakistani cricket player for Scotland. He and her mother, Khalida, purchased a home in an affluent neighborhood and had four children, who went to the nearby prestigious private school Craigholme.
Aqsa listened to Coldplay and read Harry Potter books, her parents said.
A CNN reporter saw a dog-eared copy of "The Hunger Games" sitting next to her bedroom desk.
"She was the best daughter you could have," her father told CNN in an exclusive interview last September. "We just don't know what happened to her. She loved school. She was very friendly. I have never shouted at her all my life, all my life."
She didn't seem to have any extremist beliefs, her parents said. But when civil war erupted in Syria, Aqsa became more concerned about the violence. She started praying and reading the Quran.
When she went to college, she gave up music and childhood fiction. But her parents say they were not alarmed. They said she was still going out to dinner and to the movies with her sisters.
Khalida Mahmood said that her daughter never liked to fly. The teenager couldn't stand it when people shouted.
The idea of her daughter flying to Syria to be with arguably the most brutal terror group in the world floored her mother.
On her Tumblr account, Aqsa encourages others to be strong for ISIS because the rewards are many.
She explains that ISIS wants "ghanimah" or what was considered, in the early seventh century, to be treasure taken in battle like prisoners, weapons and other goods.
As a reward for allegiance, ISIS loyalists receive gifts from Allah including "a house with free electricity and water provided to you due to the Khilafah (the caliphate or state) and no rent included."
"Sounds great right?" she writes.
Even better, followers should be get ready for "an even BIGGER reward in the Aakhirah (afterlife)."
That first phone call
Leaving family will be hard, she warns.
"The family you get in exchange for leaving the ones behind are like the pearl in comparison to the Shell you threw away into the foam of the sea," another post reads.
The "first phone call you make
once you cross the borders is one of the most difficult things you will ever have to do. Your parents are already worried enough over where you are," she writes. "... However when you hear them sob and beg like crazy on the phone for you to come back it's so hard. ... I can never do justice to how cold hearted you feel.
"But as long as you are firm and you know that this is all for the sake of Allah then nothing can shake you inshaAllaah (God willing)."
A parent with "little Islamic knowledge and understanding" won't be able to "comprehend why their son or daughter has left their well-off life, education and a bright future behind to go live in a war torn country," the teenager writes.
"Most likely they will blame themselves, they will think they have done something," the post reads. "But until they truly understand from the bottom of their heart that you have done this action sincerely for Allah's sake they will live in hope that you will return."
Parents "might assume" that their teen is going through a "phase," the post reads. She knows people who have been "here for over 2 years and their parents still try to persuade them to come back and live in false hope."
The post goes on to to give some ominous information about the possibility of recruiting teens and then, in turn, getting their parents later.
The writer says she knows parents who were angry at first that their daughters joined ISIS but who "are making plans to come visit" their daughters and "this beautiful land."
In May 2014, a long post appears on Aqsa's Tumblr. It begins, "Feeling ungrateful to your mother?"
"I'm not writing this because it's mother's day or whatever they call it," the post reads. "I am writing this because I miss my mother, and I want this to be a reminder to all of you, to recognize the worth and value of your mother, because once you lose her, nothing will be the same again.
"While most of you can still see your mother's smile, I cannot anymore. While most of you can still put your head on your mom's shoulder, I cannot anymore.
"While most of you can still call out to your mother when you feel pain in your body, I cannot anymore. While most of you can still go and have that heart to heart talk with your mother, I cannot anymore."
Heartbroken, angry parents
Aqsa's parents are angry, disappointed, confused. Their attorney, Anwar, told CNN that he got into a "blazing row" with security services because, he said, the family continues to learn information about their daughter's location and activities not from them but from media reports.
The couple will no longer engage with police unless that changes, the attorney said.
He also said that he is shocked that the missing teens were apparently not on a national terror watch list, particularly because, he said, they have had some communication with Aqsa after she left for Syria.
Back in September when CNN sat down with Aqsa's parents, they said they occasionally hear from their daughter. She reaches out to them online.
But the family isn't talking to reporters, at least right now, because they're angry about what they perceive as a lack of communication with authorities, their attorney told CNN on Monday.
For now, the only hints of Aqsa's next move come from her social media presence. There's no apparent clue on her Tumblr page that she could change her mind.
Is she haunted by her mother's urging that she come home?
When CNN spoke with her parents, her mother pleaded with her daughter: "Aqsa, my dear daughter, please come back. At this moment, I am missing you a lot. Your sisters and brother miss you a lot. My dear daughter, in the name of Allah, please come back. I miss you a lot. I love you. I love you, my dear daughter. Please come back."
Police have not indicated whether Aqsa will be subject to criminal charges if she returns.
All her family has now is hope that they'll be more convincing than the kind of people she has apparently listened to for quite some time.
On her Tumblr, Aqsa quotes Anwar al-Awlaki
, an American-born Muslim scholar, cleric and militant who acted as spokesman for al Qaeda in the Arabian Peninsula.
He was considered especially adept at recruitment. In September 2011, a CIA drone strike killed him in Yemen.
"Change," he said, "always in history, depends on the youth."