At least 100 schools in England are pressuring parents or demanding they contribute financially to budgets, potentially in breach of the law, research suggests.
The British Humanist Association (BHA) found many were state faith schools.
Schools in England are permitted to seek voluntary donations from parents, but must make it clear there is no obligation to contribute.
The government said any claims of rules being breached would be examined.
It said schools could ask for voluntary contributions but they must make clear to parents that "there is absolutely no obligation for them to pay".
A Department for Education spokesman added: "The School Admissions Code is clear that schools must not request financial contributions - whether voluntary or compulsory - as any part of the admissions process, including when offering a place."
BHA researchers carried out a simple web search to identify potential breaches of the rules and then looked in depth at the first 100 schools that came up.
They said these were dominated by faith schools, many of which are required to pay a share of building maintenance costs.
One Church of England primary, highlighted by the BHA, asks parents for an annual payment to what it calls its "Building Fund/Capitation for parents".
"This is not a voluntary contribution but it is a payment all Church of England schools require to maintain the school buildings and classrooms," it says.
Another school - a Catholic primary - says: "We will be asking all who have not contributed to the Building Fund yet for their contribution and writing to them at the beginning of next half term."
A third school explains that "as a voluntary aided school, parents of the pupils... are responsible for contributing 10% towards all building works", before going on to ask for £100 per family.
A number of schools also stressed that the requested contribution, far from being voluntary, was a minimum amount, encouraging families that could afford to pay more to do so.
Another school suggests parents should contribute to the fund by using the money they were saving as a result of receiving free school meals.
BHA chief executive Andrew Copson said: "It is simply outrageous that state-funded schools are demanding money from parents in this way.
"What's incredibly worrying is that there are evidently many more schools doing this than the 100 we found in our analysis, so we'll certainly be writing to the government to alert them to the extent of the problem."
Paul Barber, director of the Catholic Education Service, said voluntary contributions were commonly found in all types of schools.
He added: "We expect all Catholic schools to follow the Department for Education guidance on voluntary contributions.
"Parents are welcome to contribute towards their child's school and this is money spent on improving the quality of education provided."
A Church of England spokesperson said: "The admissions code is very clear about financial contributions playing no part in the admissions process and our expectation is that all schools adhere to the code."