Scotland's largest teaching union, the Educational Institute of Scotland (EIS), has voted to campaign for the removal of charitable status from the independent sector - where some of their fellow members work.
It follows an investigation by the Office of the Scottish Charities Regulator (OSCR) into how effectively the sector opened its doors to poorer pupils.
Virtually all schools have now been approved as charities and the sector was keen to move on from scrutiny on their status, which is worth millions of pounds in tax breaks.
There is concern among EIS members especially as some schools charge more than £22,000 a year for non-boarding pupils.
John Dennis, from the union's Dumfries and Galloway local association, said: "Private schools pay no domestic rates, which saves millions every year which would have gone to local authorities.
"We need to campaign to end charitable status and stop giving them these financial advantages. In reality they are operating as businesses to give a private education to the minority that can afford it."
Tom Tracey, from the Inverclyde local association, added: "They call themselves independent schools because they want to be independent because they want to be superior and because they think they are better than us. If they want to be independent then they should be fully independent."
However, Ken Bryce, from the Edinburgh local association, said he was more relaxed about the status of private schools.
He argued it would be better for the EIS to campaign for tax breaks for the state sector to level the playing field rather than attacking private schools in a campaign which would end up "trying to get rid of private schools altogether".
A motion to campaign for the removal of charitable status was carried by a majority at the EIS annual conference in Perth last week.
Following the OSCR investigation many school were deemed to have passed the "public benefit test" but others such as Hutchesons' Grammar, Glasgow, Merchiston Castle, Edinburgh, St Leonards, St Andrews and Lomond School, Helensburgh were warned they needed to do more. As a result, many scrapped traditional scholarships for talented pupils in favour of means-tested fee relief.
In April, private schools launched a strong defence of their contribution to Scottish society and the wider economy.
A report by the Scottish Council of Independent Schools (SCIS) said the value of the sector to the wider economy was £446 million a year and more than 11,200 jobs.
An accompanying commentary by John Edward, director of the SCIS, attacked the "old assumptions" about the socio-economic background of private pupils. More than 400 pupils receive a 100% remission on school fees, in addition to an expansion in means-tested bursaries.