Almost a fifth of parents have been asked to make a regular financial donation to their child's school, a survey suggests.
In some cases, mums and dads have been asked to contribute more than £100 a year.
The findings, in a poll conducted by the NASUWT teaching union, also shows that many families are handing over money to pay for visits such as trips to museums or the theatre, and some are being asked to cover the costs of activities required as part of an exam course.
General secretary Chris Keates argued that parents are facing escalating costs linked to their child's education.
The findings come amid concerns from school leaders and teachers about growing funding pressures in England's state schools.
In total, 18% said that they had been asked to complete a standing order or direct debit for a regular donation.
Of these, around 23% said the school had said it was to go to the school budget, more than half (51%) had been told it was to enhance resources, a further 18% said they had been told it was to improve extra-curricular activities and 9% said the money was going to a specific school project.
The parents polled were asked how much the donation had cost them weekly, monthly or yearly.
Of those who gave the figure as a yearly format, 61% said that they had donated £10 to £50 a year, while around a fifth (20%) said the contributions cost £101 or more a year.
The survey, published as the NASUWT meets for its annual conference in Manchester, also found that among parents who have not been asked to make a regular contribution, 13% said that they have still made a financial donation because the school expects them to, while 16% said they gave voluntarily. The rest had not made a donation.
Overall, around 96% of those polled said that they had paid for a visit related to a particular topic or subject their child was studying, while 23% had paid for an activity required as part of an exam course or qualification.
Just over a third (38%) said they have made a contribution for their child to take part in after-school activities, lessons or clubs outside formal childcare.
Ms Keates said that "substantial financial pressures" are now being placed on parents and carers.
"The costs of attending some schools are now acting as a barrier to parents accessing their school of choice for their children and are effectively a covert form of selection," she said.
"The NASUWT is clear that access to education must not be based on parents' ability to pay."
A Department for Education spokeswoman said: "No parent is required to make a contribution to their child's education, the rules are clear on this and no policies have been introduced by this government to allow schools to charge parents.
"School funding is at its highest level on record at almost £41 billion in 2017-18 – and that is set to rise, as pupil numbers rise over the next two years, to £42 billion by 2019-20.
"We recognise schools are facing cost pressures and will continue to provide support to help them use their funding in cost effective ways. This includes improving the way they buy goods and services and our recently published School Buying Strategy is designed to help schools save over £1 billion a year by 2019-20 on non-staff spend."
:: The NASUWT survey questioned almost 4,000 parents in December and January.