She is suffering from depression and seeking help from Dr Tiffany Lee Mei-yan, a clinical psychologist working at the Whole Person Development Institute. The institute, a private organisation, helps to improve people's lives.
Lee says: "Talking to her, I found she had been bullied by her classmates for quite some time. They would criticise her with harsh comments, not only verbally, but also on Facebook. She became depressed over time... she couldn't cope with the situation."
Depression is common among Hongkongers, says Lee. It is estimated that one in five people suffers from some level of depression. The problem is linked to a chemical imbalance in the brain due to a lack of serotonin which contributes to a person's feeling of well-being.
Known as a "happiness hormone", serotonin is actually a neurotransmitter in the nervous system, rather than a hormone. Although some people are born with serotonin deficiency and more prone to depression, Lee says the condition is triggered by something else - stress. In Sally's case, she could not cope with the stress caused by being bullied.
Lee says it is important for adults to recognise the signs of depression among teenagers at an early stage before things gets worse. Sally's mother regrets not seeing the signs in her daughter earlier, Lee says.
"If someone has been unhappy for as long as two weeks, had negative thoughts such as feeling worthless, and lost interest in normal activities, then he or she is depressed and should seek help."
Lee says relationship problems at home or school, and study pressure are the two major causes for stress among teenagers.
Yurie Kira, 15, a student at Kiangsu Chekiang College International Section (KCCIS), in North Point, says dealing with her friends' conflicts is stressful. "To avoid quarrels, I always have to be careful and sensitive to [their] reactions, even if I don't like what they do." When she is stressed, she can't sleep properly, and this affects her daily life. "Sometimes I feel sleepy in class and can't keep up with my studies, which gives me more stress."
Her classmates Sebastian Cheung Chun-ting and Megumi Ko are stressed about their studies. "I worry about exams and my academic performance. I have a lot of pressure from my parents and school. I can't sleep at night, then I can't concentrate in class during the day because I'm so stressed out," Sebastian, 17, says.
Megumi, 16, says: "I feel very tired from studying for exams and doing homework. Sometimes, I'm so tired I end up doing nothing. Stress gives me a skin allergy ... my body itches all over."
Coping with stress is the key to avoiding depression, Lee says. Some useful strategies include regular exercise, and adopting relaxation techniques and positive thinking, she adds.
But few students know how to cope with stress, Dr Vinci Ng, a school counsellor at KCCIS, says: "I've seen an increasing number of teens using online games to try to cope with stress. But this turns into internet addiction and leads to conflicts with their families, and so leads to more stress."
Ng says some students may even turn to smoking, alcohol or drugs. Some also tend to talk with friends, rather than their parents or a counsellor, to help ease their anxieties.
"Their peers might not have the knowledge to provide proper guidance. Therefore, friends should guide [the sufferers] to the right person," says Ng.
"I encourage students to develop new perspectives... they must have the will to take control of their problems - before the problems take control of their mind and body."
Additional reporting by Junior Reporter Fong Hui-yi