Parents may be pumping more than pound;6.5 million into schools each year - much of it to buy basic supplies such as paper and pencils.
They inject more than pound;100 per child into some schools, with primaries far more reliant on the extra money than secondaries.
The issue was first highlighted by The TESS in November, when the Scottish Parent Teacher Council launched a survey of members on their fund-raising for schools.
The findings, published this week, reveal that an average pound;10 per pupil is contributed each year by parent-teacher associations and parent councils: pound;6.5 million in total.
While 20 per cent of the money raised goes towards activities and outings, the same proportion is spent on classroom materials and books. Some 18 per cent is spent on computers and electronic goods.
"Traditionally, parent fundraising has provided funding for the extras in schools, things like special outings and events," said SPTC executive director Eileen Prior. "But it can be drawn from these figures that parents are often footing the bill for equipment that could be considered core."
In a small number of schools parents are putting in more than pound;100 per child, and 8 per cent are raising more than pound;50. The difference between primary and secondary schools is stark: primary parents raise pound;14.87 per pupil, against pound;2.41 in secondaries.
The survey highlighted "confusion" about what elements of school budgets were devolved, Ms Prior said, and many respondents were aware of cuts but unsure how severe they were.
Parents at November's SPTC annual conference said they were no longer fund-raising for "extras", but basic supplies such as paper, pencils and textbooks - a picture supported by an Educational Institute of Scotland report last week showing that some schools were struggling to pay for jotters and photocopying.
Some delegates at the conference felt schools in affluent areas were hit harder, since it was assumed parents could make up shortfalls.
It was an "unedifying spectacle" to see parents and teachers paying for essentials, said Labour education spokesman Des McNulty, who argued they were footing the bill because "authorities, under the SNP's concordat, can't make ends meet".
Education Secretary Michael Russell praised parents' generosity as a "long-standing positive aspect of Scottish education", and said the SNP had given more to councils than Labour ever had.
Some 265 responses were received by the SPTC, from schools in all but one local authority, representing 15.6 per cent of members.