A Church of England school is under investigation for wrongly requesting cash for lessons amid claims that children as young as eight were pressured into making parents pay.
Governors at St Petroc's primary school in Bodmin are investigating a complaint that letters sent out asking for money for state-funded swimming lessons breach the Education Act.
Parents have contacted the Western Morning News claiming that teachers at the voluntary-aided school have told children that they "must" bring in the weekly charge of £3.50 or they will not be allowed to swim.
One mother, who asked not to be named, said she had been forced to make an official complaint to the local education authority, Cornwall Council, to get a refund when her daughter was ill.
She said: "My daughter becomes extremely upset if I do not make the 'voluntary' payment – when I ask why she tells me that she will get told off.
"I abhor this type of emotional blackmail and threatening behaviour towards young children."
The Education Act stipulates that under the national curriculum for physical education children must be taught how to swim for 25m by the age of 11.
The Department for Education (DfE) said schools shouldn't be charging its pupils for swimming lessons.
A DfE spokesman, who was shown a letter from St Petroc's, added: "If the school does ask for payment it needs to make clear that it is a voluntary contribution – and that children will not be stopped from attending swimming lessons if payment has not been made.
"It appears, in this case, that the school has not made it clear to parents that any contribution towards the costs of the swimming provision is voluntary."
The parent who made the complaint is asking head teacher Stuart Renshaw to offer to reimburse parents as much as five years' worth of contributions.
The father, who also asked to remain anonymous, says he knows personally of eight parents who were unhappy with the requests.
He also claims Mr Renshaw told him in a face-to-face meeting that he needed funds to buy iPad computers for the school.
The parent added: "I hope that the school immediately stops involving the children in the request for any money – it is unfair for a child to feel the odd one out because they have no money to hand over to the teacher.
Mr Renshaw, who sent out the letters, has refused to comment while the matter is being investigated.
Sue Green, director of education at the Diocese of Truro, said she had "absolute faith" in the board of governors to make sure requests are "more clearly communicated in future".
Mrs Green, a former head teacher who oversees the church-funded schools in Cornwall, said schools should have a "charging policy" which is laid out to parents.
She said many schools wanted to offer more than the basic provision, adding: "I don't know of a school that doesn't ask for voluntary payments for swimming to offer more than the statutory requirement."