"It was completely unexpected. It felt so weird that I am still trying to register it," says Ashley, 16, who attends the French International School. "I was very excited."
The IGCSE is the world's most popular international curriculum. It is taken in more than 100 countries.
Apart from topping Chemistry, Ashley topped other Hong Kong students in Geography and Spanish. She also had the highest overall score across nine subjects among Hong Kong students.
Ashley admits she is a perfectionist who strives for the best.
She fell in love with the study of atoms and particles because she sees how it relates to daily life. "It's completely relevant," she says. "Take medicine as an example: All drugs are made up of chemicals, which are made up of compounds of atoms and how they relate to each other."
Classmate Cheung Tung-chit's passion is Latin, and he was the highest achiever in the subject in Hong Kong.
"Many people think Latin is a dead language but it's not," he says. "It is the root of many other languages, including English and French."
The 17-year-old is fascinated by Roman history and culture.
Both students say the school's supportive culture helps them learn. Small classes help them develop close relationships with other students and teachers. "Our classmates made a video to congratulate us and it was very nice," Ashley says.
Headmaster Francis Cauet is proud of the school's supportive culture.
"We are very close to our students and their parents," he says. "We want to make sure students feel happy and comfortable. We are thrilled to see their high achievements, but we also want them to enjoy learning." The school's achievements were also boosted by past pupil Terence Young, who topped the world in Mandarin last year. Other Hong Kong students who topped the world in the exams are Anthony Leung Chi-hin, of Sha Tin College, in maths, and Christopher Lee Pak-to, of Island School, in First Language English.
With one more year of study in secondary school, Ashley and Tung-chit already have plans for the future.
"I want to go to the US for university," Tung-chit says. "I haven't decided what to study yet, economics or psychology."
Ashley also wants to study overseas. "I like chemical engineering but I'm also interested in law. I'll decide later."