A school's new rules on riding with helmets could lead to a "fall-off" in pupils cycling in, it has been claimed.
Sandringham School in St Albans says children must wear helmets or face a ban on cycling to school, as first reported in the Herts Advertiser.
Cycling UK says helmets are not a legal requirement and schools have no power to enforce the ban.
Sandringham's head said many schools have a similar policy and it aimed to "encourage pupils to think again".
A school letter sent to parents earlier this month said the helmet rule had been introduced to "encourage good practice and uphold health and safety considerations for our students".
Parent David Stacey said: "The kids just don't want to wear them, it's a combination of 'being cool' and being obsessed with their hair.
"It's just going to deter them from cycling, which no-one wants."
Duncan Dollimore, from Cycling UK, said the group was "pro-choice" over the wearing of helmets, as it was a contentious issue as to whether helmets were effective.
He said: "It sends a really poor message to children that the school is threatening to impose sanctions with something they have no authority over.
"Where compulsory helmets have been imposed, like Australia, that led to a fall-off in teenage girls cycling to school."
Sandringham's head teacher Alan Gray said: "Many schools have this rule. We want to encourage good practice among our pupils.
"It's there to encourage them to think again."
He added that, following complaints from members of the public, the letter also said pupils would be temporarily excluded if they consistently and deliberately rode to school on pavements, which was against the law.
Mr Gray said complaints had dropped from one a day to zero since the letter had been sent.