Although the GPDST has only 26 member schools for some 20,000 pupils, it wields considerable clout as more than 90 per cent of them go on to higher education and many into top jobs.
Even nearing the millennium the trust still sees the need to continue in the spirit of its founders: to educate all girls who would benefit from an academic education whatever their background.
The trust has already made contingency plans to enhance its bursary fund to ensure that the 3,000 pupils on assisted places will not suffer if Labour wins the election and carries out its pledge to abolish the scheme.
When Labour abolished direct-grant schools in the mid-1970s, the GPDST set up the Minerva Fund, named after the trust's symbol, the goddess of wisdom, to raise money for fees for poorer girls.
In 1994, the Minerva Network was established; with 34,000 "old girl" members, it rivals, if not emulates, the "old school tie" brigade. Alison Graham, the director, said membership was spread all over the world with branches in the United States and Australia. "We're going into Europe: there will be one in Germany soon." Apart from social events, and fund-raising, members make business and career contacts.
The highlight of next week's anniversary is a "celebration of the performing arts" at the Royal Albert Hall, the birthplace of the trust. A public liability company was formed there in 1872 to establish "good and cheap" schools for girls "for all classes above those attending public elementary schools".
Two sisters, Maria Grey and Emily Shirreff, were the guiding lights. In 1850, they wrote a book, Thoughts on self culture, which drew attention to the shortcomings of women's education. Their ideas led to the formation of a national union to improve schooling for girls from all classes, which culminated in the Albert Hall meeting.
The gathering was timely. Although three girls' public schools, including Cheltenham Ladies' College, had been founded about 20 years before, they had made little general impact, as evidenced by the government report of 1869 which heralded the 1870 Education Act.
The report said: "The notion that women have minds as cultivable, and as well-worth cultivating, as men's minds is still regarded by the ordinary parent as an offensive, not to say revolutionary, paradox".
Endowments for girls' education were in "infinitesimal proportion" to those for boys, it added.
The first trust school opened in Chelsea in 1873 for 16 girls. When Blackheath followed in 1880, its curriculum was described as "dangerous" as it could only make girls "strong-minded". Trust schools pioneered new subjects for girls: chemistry in Croydon high in 1885; a science laboratory for Gateshead high in 1876; and Sheffield high was the first girls' school to have a purpose-built gym in 1884.
The founders' ethos has continued as Carol Evans, head of The Belvedere School in Toxteth, Liverpool, will testify. Founded 117 years ago, it is one of the oldest in the GPDST. "We epitomise what the trust stands for as we have an enormous cross-section of pupils. One-third are on assisted places; for many that means nil contribution from parents. We are high academic achievers - 99 per cent go on to university - and the school is renowned for being happy and friendly.
"We aim to produce the 'trust girl', although that is difficult to define. We give them the opportunity to achieve in their chosen area - we encourage them to 'have a go'. We hope the girls will leave as confident, well-rounded individuals who will make a contribution to society."
Like her colleagues, Mrs Evans views the future of the GPDST with confidence. Belvedere has expanded by 100 places to 650 over the past five years. A nursery for three to four-year-olds was introduced last year and is heavily oversubscribed.
Some of her pupils will be taking part in the Albert Hall concert on March 18 "in the choir, orchestra, and 25 are doing the Charleston".