About 150 children and parents have marched to save their Essex school from being forcibly taken over by an academy trust, in one of the largest protests of its kind.

Five- and six-year-olds waved placards proclaiming “Hands off Waltham Holy Cross” and declared it “the best school in the universe”. At other times, they sang a rough approximation of the Baby Shark theme tune, but with new words: Save our school, doo-doo-doo-doo.

Bringing up the rear was a Mr Whippy ice cream van. Its driver, Rob Thompson, has sold lollies and cones at the school gates for 34 years and had turned up in Sunday afternoon’s non-ice cream weather to show his support. “It’s such a lovely family school,” he said.

Bemused police officers freely admitted that the quiet market town of Waltham Abbey had rarely seen a demonstration before.

It was a mark of the fierce loyalty aroused by the school, which has taught generations of residents yet also faces transfer within weeks to Net Academies, a trust that has no links to the town.

Quick Guide Forced academisation of schools Show What is forced academisation? “Forced academisation” describes how a state school in England is compelled to change its legal status from a school overseen by a local authority to that of an academy, and to accept new management by an academy trust. How is it triggered? Forced academisation is an order issued by the Department for Education (DfE). A school is forced to become an academy if it is “eligible for intervention” under law. The order is triggered by a school being classed as inadequate by Ofsted. Previously it could also be triggered by poor performances in exams but that condition was dropped by the education secretary, Damian Hinds, in 2018. Do parents get any say in the matter? No. Governing bodies, parents and councils get no input in the DfE’s intervention to force academisation. They also have no say in which trust the school is forced to join, meaning the chain may be based hundreds of miles away. Critics say this is a derogation of local democracy. What happens to the school? The school’s legal relationship becomes a contract between the trust that manages it and the DfE, cutting ties with local authorities. The school’s land and buildings are effectively leased to the trust. Head teachers are stripped of their autonomy, with budget and staffing decisions made by the trust. In most cases the school’s existing leadership is dismissed. The school’s governors lose legal responsibilities, and there is no requirement for trusts to consult with parents. Often the school is renamed and a new uniform adopted. The trust retains a proportion of the school’s funding for its administration and executive costs. Does forced academisation improve schools? There is little evidence either way. Previously, three-quarters of schools rated as inadequate by Ofsted later improved without forced academisation. The National Audit Office has concluded there is a lack of capable Mats able to improve schools in difficulty.

Since March 2018, when Ofsted inspectors judged Waltham Holy Cross to be failing, all the big decisions over the school’s future have been made behind closed doors, miles from the scene of this rally. A Whitehall official decided that it should be handed to Net Academies, which has run schools from Warwickshire to Reading, before expanding into Essex. Both that decision and the Ofsted verdict were challenged by the school’s senior staff, to no avail.

When told that their school would be under new management, parents were not informed that two Net Academies schools elsewhere have been ranked “inadequate” by Ofsted, nor that a league table published last summer ranked the trust as the sixth-worst primary group in England.

“Why should Net take over this school, when they’ve shown they can’t run others?” asked Vanessa Brenchley. She had attended the school as a girl and lives on its doorstep but declared that any takeover by Net would force her to send her son, James, elsewhere. “I’ll drive miles if I have to.”

The march was the latest in a series of sometimes unorthodox actions taken by local parents, who now dub themselves “the accidental activists”. To uncover details about the fate of their own children’s school, they have had to file hundreds of freedom of information requests, and spent many months getting on top of the obscurities of education policy, land agreements and the relationship between different branches of the state.

When one mother heard the education secretary, Damian Hinds, on a radio phone-in, they flooded the switchboard demanding to meet him.

“We are fighting an entire system,” said Jayshree Tailor, who has two children at the school.“We are up against officials who are telling us and the school that we must go from point A to point B to point C – and that we can’t ask why or deviate in any way.”

Parents at Waltham Holy Cross feel that their children’s may not be met by Net Academies Trust. Photograph: John Stillwell/PA

Others worried about how their children would fare under a new regime. Lauren and Ben Alston had heard from parents at other schools run by Net how their children with behavioural or other problems were liable to be excluded. Their six-year-old son, Thomas, is autistic and his mum described how just last Friday he’d been too anxious to face school – so the form teacher had come and met the pair outside and walked him in. “That’s not going to happen if Net take over,” she said. “They’ll just offroll him.”

Also part of the march were parents and staff from other school battles against forced academisation. Travelling from south London, teacher Kirstie Paton had navigated the disruption of the marathon to march alongside with a banner for her own secondary school, John Roan. She is in regular contact with the parents of Waltham Holy Cross through a WhatsApp group for school campaigners across the country, who swap news and strategies. “You can go away for a couple of hours and come back to 450 pings,” she said.

Sunday’s rally ended in the market square with a raffle of prizes donated by local businesses and shops, awarded by the local mayoress who’d hobbled on crutches alongside the parents. An adjoining table groaned with two large cakes cooked by another friend of the campaign. In blue icing, one read Hands Off WHX.

In the April drizzle, stonemason Shaun McDonnell talked of how the school had helped his son, Aiden, who has attention deficit disorder. He had never been on any demo before, but there was a first time for everything. He said: “Public opinion’s got to matter sometimes, surely?”