Its 60 staff allocate budgets and provide legal and personnel advice and in-service training to its schools. It owns their buildings and grounds and pays for major repairs and improvements, while leaving broken windows and redecoration to the schools. It employs the staff, sets pay scales and plays a major role in appointing headteachers, but otherwise leaves both appointments and curriculum to the heads and governors. If a school needs to get rid of a teacher, it will oblige.
But this is no local education authority. It is the Girls' Day School Trust (GDST), which owns and runs 25 of the most academically successful independent girls' schools in the country.
The trust's central administration means that its schools also have some of the lowest fees: less than pound;1,900 a term in London, less than pound;1,600 elsewhere.
The idea of creating a network of good yet affordable day schools for girls was first put before the public at the newly opened Royal Albert Hall in 1872. The prime movers were two sisters, Maria Grey and Emily Shirreff, supported by the dowager Lady Stanley of Alderney and Mary Gurney. They set up a limited liability company to raise capital and decided to establish schools where locals were interested enough to buy shares and fund the purchase of buildings. The first opened in Chelsea in 1873.
The Trust's schools have a long history of partnership with the state. In 1944 they joined the Government's direct grant scheme but, preferring to remain academically selective, opted for full independence when the scheme was discontinued in 1976. From 1981, the Trust was the largest participant in the Assisted Places Scheme, with more than 3,000 of its 19,500 pupils on the programme.
When Assisted Places was phased out from 1998, the GDST launched an appeal to set up its own bursary scheme. It has so far raised pound;20m and this year offered some 500 bursaries - nearly a quarter of the new intake.
The new bursary means test has been carefully devised to avoid the abuses of the old Assisted Places Scheme and takes both financial capital and "local knowledge" into account. "We know if there are two Range Rovers in the drive," says Michael Oakley, the GDST's secretary.
Now the Trust is entering a new partnership, this time with millionaire entrepreneur Peter Lampl's Sutton Trust. From next autumn, the two trusts are jointly footing the bill to turn the Belvedere, a GDST school in Liverpool, into the country's first open access independent day school, with admission purely on academic merit, regardless of ability to pay. Applications are flooding in.
Looking to the future, the GDST is considering whether to embrace girls' grammar schools that would otherwise go comprehensive following local parental ballots. But Mr Oakley says the trust could only act if the school owned the property, restricting the field to charitable trusts.