Four in 10 parents are being asked to contribute regularly to school funds in England, Wales and Northern Ireland, a survey suggests.
Schools have often asked parents for contributions to a school fund at the start of the year, the Parent Teacher Association UK said.
Its survey of 1,507 parents found a third regularly gave to school funds in the last year.
The Department for Education said no parent was obliged to contribute.
It added that schools were forbidden from charging for education or materials provided during school hours.
But they are well within their rights to ask parents to make voluntary contributions.
Those that do may ask for a lump sum or for a direct debit to be set up.
They can also ask parents to contribute towards the cost of extra-curricular activities and school trips.
The PTA UK survey also revealed that more than three-quarters of parents think the cost of sending their child to a state school is increasing.
And this was a concern for more than half of schools, the survey suggested.
Parents have long contributed to schools, through financial contributions and by volunteering their time and energy.
But last year, as many schools struggled to balance budgets, parents reported receiving requests for cash, some of which specifically mentioned budget black holes.
Acting head of PTA UK Michelle Doyle Wildman said parents’ support helps give children better educational experiences.
She added: “Parents are reporting they are contributing more to provide the essentials which many expect to be provided by the state.
“If this is a growing trend, then it’s crucial that schools work in partnership with parents to address their specific concerns.”
A Department for Education spokesman said: “No parent is required to make a contribution to their child’s education.”
It stressed that there had been no change in the rules regarding voluntary contributions.
And it said its new funding formula, backed up by an extra £1.3bn, would help continue the trend of improvement in schools and replace an outdated funding system.
Jo Yurky, who campaigns for more funding for schools, said schools could not be funded “by having a whip-round”.
“We make no criticism of those schools involved – they’re desperate.
“We are calling on the chancellor to use his autumn Budget to provide a full remedy to the financial crisis facing all our schools.”