It was hoped the New Model School, created by Civitas, would take up to 27 pupils, but only ten have so far been recruited.
The school, in Queen's Park, north-west London, had been billed as a cheap alternative for parents disillusioned with the state sector, but unable to pay expensive independent fees.
The pound;900-a-term school aims to give five-year-olds a reading age of eight. French and world cultures will be taught from the age of four and yoga and martial arts from five.
It opens on September 13 for reception-age children in rented rooms in a local leisure centre. It will build up classes year-on-year. At present the school has just one teacher and lessons last only until lunchtime to keep prices down.
But Robert Whelan, deputy director of Civitas, insisted that low pupil numbers were to be expected, even though 100 parents had turned up to a project launch meeting in February. "It was inevitable that we would have a slow start-up - what is important is to build up the model so it expands in future," he said.
Shareholders have contributed pound;25,000 to the project and Mr Whelan admitted it was likely to run "at a loss" for the first year at least. But he insisted it would be viable long-term and will eventually move into purpose-built or bigger rented accommodation. He said at least two more schools are planned for next year, one in London and one outside the capital.
The Civitas project is the latest move to create a "no-frills" private schools market. The largest private school operator in England, Global Education Management Systems (Gems), hopes to run 120 schools within five years, charging parents around pound;5,000-a-year.
Dick Davison, joint director of the Independent Schools Council information service, said: "We always welcome any effort to make the independent sector more inclusive but I would like to see how schools like this can possibly operate on such budgets."
A spokesman for Brent council said its schools, 11 of which have beacon or leading-edge status, provided an excellent education.