And, in a direct reference to the Laura Spence affair, they warn that discriminating against privately-educated students when assessing university applications could undermine partnerships with state schools. Both issues are raised in a 10-point statement of principles issued by the Independent Schools Council, and directed at all the political parties in the run-up to the general election.
The council, representing 1,300 independent schools, says it is working on an alternative to the Assisted Places Scheme, which the incoming Labour Government abolished to help fund the reduction of infant class sizes.
It wants the Government to contribute up to pound;3,000 per pupil - roughly equivalent to the cost of educating a teenager in a state secondary school. This would be a quid pro quo for the pound;1.7 billion the sector saves the Exchequer by educating 600,000 youngsters, whose parents contribute a similar sum to state education via taxes.
The ISC statement welcomes government efforts to improve partnership between the maintained and independent sectors since abolishing assisted places. But it warns hat the relationship could be soured by discrimination against private school students and exclusion of 50,000 independent school teachers from state-funded developments, such as computer training.
Dick Davison, joint national director of ISIS, the ISC's information arm, said universities should not be "brow-beaten" into discriminating against private-school pupils.
Referring to Laura Spence, the comprehensive student who was turned down by Oxford University, he said Chancellor Gordon Brown's intervention in the case was "ill-informed and unhelpful", and amounted to "social engineering".
The ISC document cites legal challenges to affirmative action programmes in United States' universities, and argues that independent school teachers are still under-represented on the General Teaching Council, with only two places.
ISC chairman Ian Beer said: "Despite its abolition of the assisted places scheme, the Labour government has done a great deal to improve and foster relations between the independent and maintained sectors.
"But there remain vital areas where the position of independent schools, and those who are educated and who work in them remains either unclear or unacknowledged."
The ISC represents 1,300 schools educating more than 480,000 children.