The oldest girls' school in South-east Asia was almost never founded.

In 1842, Mrs Maria Dyer, a missionary of the London Missionary Society, was en route to China when she stopped over in Singapore. There, she stumbled upon the plight of the "mui tsai", a term in dialect referring to Chinese girls from poor families who were sold to the rich as servants.

She could have looked the other way, or written it off as a Chinese tradition. Instead, Mrs Dyer sought permission from the then governor of the Straits Settlements to start a boarding school which would house and educate these girls. This school, then called the Chinese Girls' School, later became the St Margaret's School of today.

Last Thursday, 525 guests, many of them old girls from 1924 onwards, as well as current and former teachers, celebrated the 175th anniversary of St Margaret's School at the Shangri-La Hotel. President Halimah Yacob was the guest of honour at the gala dinner.

Many of the alumnae had fond memories of their principals and teachers. Though they graduated decades apart, they agreed on this: The school staff were always kind and understanding.

"Martha Holloway (principal from 1957 to 1965) made a point to know every child in school," said Ms Lee Soo Kwan, 72, from the class of 1962. Ms Lee is also the current president of the St Margaret's Ex-Students' Association.

She related the story of a girl who was "perpetually late".

"(The principal) saw her in detention class on numerous occasions, and took it upon herself to find out why. She discovered that the student had to take her siblings to school and do household work before she herself could come to school. So she told the prefects not to send her to detention any more."

Ms Chan Lye Yong, 61, from the class of 1972, who teaches geography at the secondary school, recalled a literature teacher who was "so passionate and so animated".

Decades later, the same teacher sponsored a book prize, out of her own pocket, for students who were outstanding in that subject. The Isabel Low English Literature Book Prize, started in 2003, has been awarded to the top-scoring literature student every year since.

The girls reciprocated with their own brand of warmth. Ms Lee recalled this from her student days: "We used to have teachers-in-training coming to our school. We used to tell them, 'When your supervisors come around, we'll be very cooperative.' When questions were asked, you'd see all hands raised - but right hand meant we knew the answer; left hand meant we didn't!"

Even after graduating, that spirit remains among students.

Ms Ruby Lee Yen Kee, 55, from the class of 1978, is a member of a WhatsApp group made up of other graduates from her batch. "It's very interesting. Whoever needs anything puts it on the chat group, and someone will respond," said the associate professor from the National University of Singapore's Faculty of Law. "Two days ago, somebody needed a hospital bed and a wheelchair. Within the afternoon, someone said, 'Ah, I have a spare one.'"

On Feb 10, St Margaret's Secondary School entered the Singapore Book of Records by arranging 4,208 tins of donated canned food to form the words "Glowing In His Glory: 175 Years of Charity, Patience, Devotion". The food was later donated to needy residents from the North West Community Development Council. On the same day, the school's heritage gallery, which was closed for renovation last year, was officially reopened by Mrs Chua-Lim Yen Ching, who is deputy director-general of education (professional development) at the Ministry of Education and executive director of the Academy of Singapore Teachers, as well as an alumna.

This gallery, located on the secondary school premises in Farrer Road, showcases artefacts such as an old school bell, a duty roster, and a set of the old pinafore version of the school uniform, which was phased out after 1974 in favour of a its current design.

On June 3, the school hosted a Homecoming and Green Dot Funfair.

Finally, the gala dinner saw the debut of an online repository, where past students and teachers can submit memories about their schooldays. The project will accept submissions over the next few years.

Madam Lee Lin Yee, 39, the principal of St Margaret's Secondary, said: "I always ask my students: 'You want to do well, but for what purpose?'

"We want our graduates to not just do well, but to also do good."