Cash-strapped schools are sending parents invoices for voluntary contributions in a practice that the peak parent body says is exploiting vulnerable families who feel pressured to pay optional fees of more than $2000 at some NSW public schools.

"Schools are sending out a letter that says 'welcome back to school, the contribution this year is ...' and then they just send an invoice for the amount home," said Susie Boyd, president of the NSW Parents and Citizens Association, which is calling for an end to voluntary contributions.

Sydney Boys High School has one of the highest annual contributions of $2517 for a year 10 to 12 student in 2019

"Up to 80 per cent of the parents we have spoken to don't realise the contribution is voluntary, it's not clearly articulated at some schools and parents are taking on this burden that isn't their burden.

"Public schools should be free to all but with contributions and the rising cost of uniforms, stationery and excursions that parents also have to pay for, it's not a free education by any means."