STUDENTS at Sydney's private boarding schools are from even wealthier families than first thought. So much so, that many schools fear they will lose funding once the secret is out.

Socio-economic scores given to every school on the federal government's My School website have been recalculated this year using enrolment information instead of the census district data used for last year's scores.

Using the new measure, the scores for private boarding schools have risen by up to 15 per cent on the new version of the My School website, expected to be released by next week.

The federal government, however, still uses census district data to distribute funding.

It is believed some boarding schools attract inflated funding because the families of country students may be wealthier than where they live suggests. For example, wealthy farming families from poor rural areas often attend Sydney boarding schools.