You can't argue with the numbers.
From The Evening Standard:
Richard Harman, chairman of the Headmasters’ and Headmistresses’ Conference of leading private schools, said [private] schools add nearly £12 billion to the UK’s GDP every year.
"More than £4.7 billion in tax revenue flows annually into the Exchequer from the direct activity of independent schools. And by educating our pupils we save the taxpayer £3.9 billion a year, equivalent to building more than 590 new free schools annually."
Mr Harman said: “We have solutions to offer. But too often those in power are embarrassed to be seen talking with us, preferring instead to threaten us with loss of charitable status or more state control."
You can quibble with them though. If anything I think his £3.9 billion saving to the taxpayer is understated and expressing the total figure in terms of the number of "free schools" which could be built is a bit meaningless. But his general point stands.