Millions of families across Britain are struggling to meet the hidden costs of state education, suggests research carried out by young people.
The cost of non-core items risks pricing poorer pupils out of some subjects, their report claims.
The Children's Commission on Poverty says basics, such as uniforms, school trips, materials and computer access can amount to £800 per child each year.
The government said it was already trying to help disadvantaged children.
The £800 could include the likes of £159 on school uniform and sports kit, £82 on travel, £167 on school trips and an average of £168 on school meals, says the report by 16 young commissioners, aged 10 to 19, appointed by the Children's Society.
'Embarrassed and excluded'
These costs can mean disadvantaged pupils miss out or feel stigmatised, say the authors.
"The impact of school costs is leading to children feeling embarrassed, bullied and excluded because they cannot afford the same things as their peers."
More than half (52%) of almost 2,000 parents surveyed for the report said they had had to cut back on clothing, food or heating to pay school costs - a figure that rose to 95% among those who defined themselves as "not well off at all".
Some students find subjects like art and technology prohibitively pricey, says the report.
One teenager told the commissioners: "It got progressively more tough... We needed to pay for things like my sketchbooks for art and tech. The money was being stretched quite far and I started realising then that I couldn't keep asking for those things."
Some of the poorest said they lacked home computers and internet access, which are increasingly required for homework, and some teachers were unsympathetic.
One student said: "I keep telling [the teacher] I didn't have a computer, and then he just kept shouting at me, and I had to say out loud that I didn't have a computer, and everyone started laughing."
Another complained of being given detention because the website prescribed did not work on their computer.
The authors urge schools to "poverty proof" course materials and trips by limiting costs.
They want schools to avoid elaborate uniforms, that can cost up to £500 from specialist suppliers, in favour of basic supermarket alternatives for about £40.
They also urge the government to widen entitlement to free school meals to include low-income working families.
Russell Hobby, general secretary of the National Association of Head Teachers, welcomed the report's insight "into some of the unintended consequences of school policies on the growing number of children living in poverty".
Schools Minister David Laws said: "The coalition is building a stronger economy and a fairer society, where everyone has the opportunity to succeed whatever their background."
Mr Laws said the government's £2.5bn pupil premium for disadvantaged children was closing the gap between the poorest and their peers.
"We are also determined to help all families with the cost of bringing up their children. We have cut income tax for millions of ordinary working parents, and introduced free school meals for all infant children.
"These measures will improve the prospects of all children so that they have the best possible opportunities in life - and this will continue to be a top priority."
The evidence, gathered and analysed by the young commissioners with help from the Children's Society, included three hearings in Parliament alongside MPs and peers, as well as written submissions from expert groups and in-depth interviews with families.