Class sizes in the independent sector must increase to keep fees under control and secure the future of some schools, research says.
Teacher-pupil ratios have fallen over the past 10 years, with average classes now having fewer than 10 pupils. But higher costs mean schools now risk "pricing themselves out of their market", according to a report by accountancy firm Horwath Clark Whitehill.
The warning follows a number of inflation-busting fee rises. Average fees are up by more than 40 per cent since 2003, according to the Halifax.
It also comes as many schools are assessing their finances in light of a slowing economy and the potential impact of public benefit tests being introduced by the Charity Commission. "Pupil-teacher ratios continue to decline and . there must be concerns about the long-term affordability," the report says.
"The challenge in the future may well be to seek to provide the same quality of education, but accepting that the pupil-teacher ratio has to rise to keep the fee within affordable bounds."
There is a chance some schools will "break the mould and offer significantly lower fees with higher pupil ratios", says the report, which surveyed more than 500 schools.
Census data released by the Independent Schools Council, which represents more than 1,300 schools, shows class sizes falling from 10.8 pupils in 1999 to 9.6 last year.
Each full-point increase in pupil-teacher ratio leads to a fee rise of about pound;625, the report estimates.
But Peter Bodkin, head of pound;9,539-a-year Tettenhall College in Wolverhampton, said small class sizes were the "number one selling point" of independent schools.
"We would only go down the route of increasing class sizes as a last resort," he said. "If schools have the option, most will put off the glitzy new art block rather than sacrifice small class sizes. If I had to make a choice, I would invest in people rather than facilities."
The survey also found that the proportion of fee assistance spent on discounts for the children of staff fell slightly in 2007. Staff discounts accounted for 17.5 per cent of fee concessions, down from 17.9 per cent in 2006.
The discounts, a strong recruiting tool, are unlikely to count towards public benefit tests. This could put them at risk as schools put more money into means-tested bursaries.