The number of pupils in Britain's independent schools has risen again, to reach 508,027.
School fees have risen greatly in recent years
The Independent Schools Council annual census shows numbers were up for an unprecedented ninth year in a row.
The increase on 2003 was slight - just 0.1% - but the sector's growth has been against a flattening off in the number of pupils of school age.
Fees rose 9.6% on average - as the Office of Fair Trading investigates the way schools have shared information.
Average fees are now £3,074 a term. Boarding fees were up 9.1% to £5,909, while rates at day schools rose 10.1% to £2,429.
The census, counting the number of pupils on a given day in January, showed there were 416 more than last year.
ISC member schools account for about four-fifths of the independent sector.
The census report says parents regard smaller teaching groups as one of the main attractions of independent schools.
Fees are the price they pay for this. But almost one in every four pupils receives financial assistance from his or her school.
There were 0.3% more day pupils - who account for 86% of pupils in the sector.
The total number of boarders was slightly down, "after two years of recovery from a long decline", though spending on new accommodation and facilities rose by more than 30%, from £69m to £90.5m.
The director of the ISC's information service, David Woodhead, said: "While the overall growth in numbers is smaller than in recent years, it must be seen in the context of substantially higher fees attributable to an unusual combination of cost pressures on schools."
He mentioned a "huge increase" in the schools' contribution to teachers' pensions and a 1% rise in employers' National Insurance contributions.
Other increased costs included charges for Criminal Records Bureau checks on staff and, for boarding schools, new charges for welfare inspections.
Schools have also been investing more in buildings and equipment, spending £523m last year - an average of £1,034 per pupil compared with £984 in 2002.
The Office of Fair Trading (OFT) is investigating the way many top schools shared information about fees.
The chair of the Boarding Schools Association, Stephen Winkley - headmaster of Uppingham School in Rutland - said 52 schools had submitted material to the OFT at the beginning of March.
"It's hard not to admit that we were sharing information," he said.
"Information was shared. We didn't realise there was anything wrong about it but clearly, technically I think schools will find themselves admitting that an error was committed.
"It's well known that schools did talk to each other, just on a benchmarking basis."
The ISC says schools had quite openly continued to follow a long-established practice, unaware the law had changed.
The 1988 Competition Act replaced the 1976 Restrictive Trade Practices Act, under which private schools had been exempt from the anti-cartel rules that applied to businesses.
Boarding fees at Uppingham School are now £6,700 a term
Dr Winkley said schools' fees had tended to go up by similar amounts because their costs were very similar.
But he was "frightened" by the size of the latest increase and its potential effect on parents - though as yet his school had not seen any significant impact.
"There's no sign that it's put people off," he said. "Our books are as full as they have ever been."
Uppingham - like other boarding specialists - is investing in new accommodation, spending £2.9m building another boarding house for girls.
The school became co-educational a few years ago.
"That's been a wild success," the head teacher said. "I think I could build three more girls' houses and fill them without difficulty."
The census shows that pupil numbers from overseas rose 3.3% overall, though much of this was attributed to expatriate British children coming to school in the UK (up 29.2%).
There was again strong growth in demand from mainland China.
The ISC said new admissions from the People's Republic rose by 13.1% to 1,102.
Germany remains the largest single source of students from Europe. Just over 1,000 Germans, mostly 16-19 year-olds, came to ISC schools last year - up 11% on the previous year.