An online pledge to withhold smartphones from children until they are at least 14 has been signed by parents at 20% of schools across the UK, according to a campaign group.

A “parent pact” organised by Smartphone Free Childhood has been signed by at least one parent at 6,537 schools, with the signatories representing just under 35,000 children. There are 32,000 schools across the UK, including nurseries and pupil referral units.

The pact states that “acting in the best interests of my child and our community, I will wait until at least the end of year 9 before getting them a smartphone”.

Daisy Greenwell, the group’s co-founder, said the pact’s aimed to reduce the peer pressure factor among children asking for a smartphone from their parents, as well as pushing up the average age at which children obtain one. Greenwell said owning a brick phone – a smartphone alternative that allows text and calls only – could become more acceptable for children if they knew their peers were barred from having smartphones.

“The peer pressure instantly dissolves if your child knows there are 10 children in their class who are getting a brick phone as well – and not a smartphone,” she said.

According to research by the media regulator, Ofcom, 89% of 12-year-olds own a smartphone.

Greenwell said: “The ultimate aim is to permanently shift the culture around when you give smartphones to your children.”

Smartphone Free Childhood said the pact was anonymous but it has broken down data on sign-ups into regions and schools, publishing the findings on its website on Thursday.

The most signatures are in Hertfordshire, where 2,429 parents have signed the agreement. The county is a focal point of the smartphone-free movement, with the city of St Albans attempting to become the first UK city to go smartphone free for all children under 14.

The next largest regions are south-west London, with more than 1,700 sign-ups, and Hampshire, with more than 1,500. Greenwell added that the group’s website could then direct parents who have signed the pact to relevant regional communities and school groups on the WhatsApp messaging platform.

Concerned about the impact of smartphone ownership and social media on child development, Greenwell and her friend Clare Fernyhough created a WhatsApp group in February last year to support each other in their decision to hold off from buying their children smartphones. The group now says its community comprises 150,000 parents.

Jonathan Haidt, a US academic whose book The Anxious Generation has been a key influence on the smartphone-free movement, told the Guardian this year that parents should set a “new norm” and hold off giving children a smartphone until they are about the age of 16. Most social media platforms have a minimum sign-up age of 13 but Ofcom research shows that half of children under 13 are on social media.