A primary headteacher has refused to enter her pupils for Sats tests this year, despite being told she could lose her job over the decision.
Jill Wood, who leads Little London Primary School in Leeds, said the move followed extensive discussions with parents and governors.
She told Tes that the key stage 2 test results would not compare "like with like" this year, after national curriculum levels were removed last year as a measure of pupil performance, allowing schools to design their own assessment systems.
"I'm not against testing," she said. "We're accountable, and we're robust in the way we measure our children's progress. I believe I'm a public servant.
"We're not trying to wrap them up in cotton wool. But how can we ask children to do standardised tests when nothing is standardised anymore?
"When we were measuring [levels] it made sense to do that, but then the government asked us all to go away and devise our own way of measuring our children's progress. There's nothing standardised."
'I'm not trying to start a revolution'
Ms Wood said she was also concerned about the impact of the tests on children's wellbeing, and the fact that they were taken at a point in the school year when there was still more than two months of learning left.
A cross-party committee earlier this month warned that the way primary schools are held to account for Sats results was harming children's learning and teacher and pupil wellbeing.
Ms Wood said she had been told by her local authority that she was in breach of her employment contract and could face repurcussions from Ofsted. Her school will be subject to a whole school review by the local authority, and is also included in the key stage 2 moderation process.
She has also received "abusive messages" from members of the public accusing her of "turning pupils into wimps", she said.
She added: "I'm not trying to start a revolution, we were just quietly getting on with not doing the tests." She would consider entering her pupils into the tests next year if the assessments are changed, she said.
The vast majority of heads rejected a Sats boycott in a survey of members of the NAHT headteachers union earlier this month.
An NAHT spokesman said the union would not advise its members to take children out of Sats testing.