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Pity state schools can't be charities.
Most state schools have a PTA which is a charity. The PTA raises money and buy things for the school to lighten the load on the school. The PTA can't pay salaries but if the PTA funds, for example, the resurfacing of the playground, then the school doesn't have to pay that out from its own money. In reality, and given the swingeing cuts to school funding, I expect some schools are using PTA money to buy essentials such as books/paper/etc.
It varies hugely around the country though (unsurprisingly). The PTA at my daughter's old primary school (single form entry, so ~210 children) used to raise close to £50k a year. Most of it was from donations (including gift aid) and employer matching, secondary were general fundraising events.
[EDIT] And it's not just because it's an affluent area (but it obviously helps). One of the other primaries in the same area, with a very similar profile of parents, raises about £2k a year as they don't put much effort into fundraising at all. Having seen the PTA side of things it takes a metric fuckload of effort to raise big chunks of money, but it does pay off.
Don't forget Academies :/