The cost of new transition events to smooth entry to Year 7, such as overnight stays in new schools, is also placing a huge strain on family finances, suggests research from the End Child Poverty Network Cymru.
It estimates the average school trip costs pound;186, and says many children from poor families don't even bother taking letters home because they know their parents will not be able to afford them.
Other hidden school costs - including textbooks, specialist computer software and after-school activities - are disadvantaging poor children.
Poor families in the south Wales valleys, where only 26 per cent of homes have internet access, may also face extra transport costs for children having to undertake online research projects after school.
Sean O'Neill, network co-ordinator in Wales, said: "It's a shame that going to school has the potential to isolate, exclude and stigmatise children."