The independent Schools Council has criticised the Charity Commission for naming the first schools that are to be scrutinised under new public benefit tests.
The ISC, which represents more than 1,200 schools, said the commission had been wrong to publish the list of five schools before finalising its guidance on what will be expected of the sector.
The commission named Manchester Grammar School Foundation, Manor House School Trust, Pangbourne College, St Anselm's School Trust and High Priory School as the first to be assessed.
They will have to fill out detailed questionnaires to prove their public benefit credentials and can expect to be visited by the commission before Christmas.
Under new laws, independent schools will have to show that they are accessible to children from poorer backgrounds in order to retain their charitable status and tax breaks, worth around pound;100 million a year.
Private schools are also likely to come under increased pressure to create partnerships and share their facilities with state schools.
The commission's finalised guidance for the sector is expected before the end of the year but has not been published yet.
David Lyscom, the ISC's chief executive, said he was confident that the five schools would pass the tests.
"We believe the commission's reviews will demonstrate that schools are carrying out one of the oldest charitable missions - advancing education - and doing so more or less at cost price," he said.
"Faced with current cost pressures, we believe it is all the more remarkable that most schools will also be able to demonstrate that they make exceptional contributions via free or discounted access, sharing arrangements and community partnerships."
ISC schools already donate more than pound;350m each year in fee assistance to 125,000 pupils, Mr Lyscom added.