Teachers at a school which has been at the centre of a parent-led campaign against plans to hand it to an academy trust are to hold four days of strikes beginning next week.

Industrial action at Waltham Holy Cross primary in Waltham Abbey, Essex, was announced as it emerged that the school had been referred to the government’s Standards and Testing Agency (STA) over concerns about the administration of recent Sats testing of six- and seven-year-olds at the school.

Teachers conducting the extended writing assessment in question were being guided by planning materials from the incoming trust, NET Academies, before the trust takeover of the school, according to the National Education Union (NEU), which represents staff there.

An external moderator from Essex county council monitoring the assessment raised concerns about the level of “scaffolding” provided by teachers, indicating that pupils had been given too much help for it to be judged as independent work.

The revelation will add to parents’ concerns about the suitability of the trust to take on their school. A spokesperson for NET Academies trust confirmed that “an external moderation process was commissioned by the school, and as a result of this a referral was made to the Standards and Testing Agency.

“NET Academies trust has not been informed of any action to be taken by the STA. NET academies schools are externally moderated on an annual basis. All moderation visits have supported NET judgments.”

The school is due to be formally taken out of local authority control and turned into an academy after a failed Ofsted inspection in March 2018. Parents who oppose joining the trust have spent months campaigning, filing hundreds of freedom of information requests and turning themselves into experts on the detail of education policy.

Shaunagh Roberts, a parent at the school and a leading figure in the campaign against the trust, called for an investigation. “There’s no way we can believe their results. Why isn’t the STA taking action? It’s just shocking.”

Jerry Glazier, the NEU Essex branch secretary, added: “Clearly there’s an irregularity and that raises questions about the judgment of the trust in the way it’s operating.”

More than 20 NEU members at Waltham Holy Cross will take part in the industrial action next Thursday and then on a further three days beginning on 25 June.

Ben Collin, a joint NEU school representative, said: “Taking industrial action as a teacher is never an easy decision to make as we always place the children and their learning first.

“However, it is impossible to ignore the potential negative impact that the academisation order and the appointment of NET Academies trust as the school’s sponsor could have on our children’s futures.” A recent league table published by the Education Policy Institute showed NET was one of the worst-performing primary groups in England.

A spokesperson for Essex council said: “We are aware of the referral made by EES for Schools, the trading arm of Essex county council, to the Standards and Testing Agency as part of an external moderation that was commissioned by the school. We understand no further action has been taken by the STA following the referral.”

A Department for Education spokesperson said: “The Standards and Testing Agency has not investigated any schools within NET Academies trust.

“However, we are clear that any instances of maladministration of the national curriculum assessments are completely unacceptable.”

Waltham Holy Cross primary school have also been contacted for comment.

Further strike action has also been announced at Peacehaven community school in East Sussex, where teachers and parents are fighting plans to hand the town’s only secondary school to the Kent-based Swale Academy trust.

Teachers are to hold seven further days of strike action at the end of this month and into July. A letter signed by the Labour MP Lloyd Russell-Moyle, Conservative MP Maria Caulfield and 12 Peacehaven town councillors from different parties has called for a ballot of parents before the school is taken out of local authority control.

The letter to the East Sussex county council leader, Keith Glazier, raises concerns about unqualified teachers, excessive executive pay and a lack of accountability in academy trusts, the NEU said.

Parents and teachers at two Peacehaven primary schools recently successfully fought similar plans to turn their schools into academies, but the future of the secondary remains in doubt.

Phil Clarke, the local NEU secretary, said: “The letter from local politicians across parties is very welcome as it is asking why Peacehaven are being denied the same choice and democratic process afforded to other schools and communities.”

• This article was amended on 24 June 2019. An earlier version said that NET Academies had been referred to the government’s Standards and Testing Agency. After publication, NET Academies contacted the Guardian to say that following its own investigation into the referral to the STA, the chair of Waltham Holy Cross primary school’s Interim Executive Board confirmed that it was in fact the school that were referred to the STA, not NET Academies. This has been corrected.