She said she was convinced the children made more progress in the two months following the tests than in the nine months beforehand, when she was having to teach them a "Gradgrind curriculum" in the test subjects - English, maths and science.
"Stop the cruelty," she said. "Boycott the Sats."
A spokeswoman for the Department for Children, Schools and Families said: "It is regrettable that the NUT leadership and their conference, in voting for a boycott, are setting themselves against the clear wishes of parents and the need to raise standards in every school and in every child.
"Not only is a boycott unlawful and causes great disruption to the schools; it also sends entirely the wrong message to children and young people an undermines the standing of the teaching profession.
"The unions representing the majority of teachers do not support the approach being urged by the NUT leadership and we urge the NUT to think again."
The other unions that represent classroom teachers - primarily the NASUWT and the ATL - have not backed the joint campaign being mounted by NUT and NAHT leaders.
Some NUT members say Sats make children nervous
The NUT's Christine Blower said they did not, however, support Sats and league tables any more than her union did.
The NUT conference is being held this year in Cardiff - where the Welsh Assembly Government scrapped the Sats in its schools between 2002 and 2005.
The NUT says this absence of testing in Wales has not resulted in an upsurge of "barbarian hordes".
However, the Welsh inspectorate, Estyn, has expressed concern that the proportion of five to seven-year-olds with good levels of reading and writing has stopped rising over the past five years.
Some of the NUT delegates held a small demonstration in the Cardiff sunshine outside their conference venue, wearing red T-shirts with the slogan No Useless Tests (NUT).
One, David Clinch from Devon, said: "Sats are like cigarettes. They've got no benefit to the human body whatsoever.
We want to scrap the Sats now. We know they're bad for children
NUT member Sara Tomlinson
"What they do is make children very nervous about their learning in fact they are not learning they are being coached to do particular tests which have no benefit to them at all," he said.
"The key benefit is to the state to make schools compete against one another and to put schools into league tables, which is not what we think, we believe in collaboration and innovation in teaching."
Another, Sara Tomlinson from Lambeth in London said: "We want to scrap the Sats now. We know they're bad for children.
"Every report, every survey that's done by expert groups says they are damaging to children."
She said there was no problem with having a bank of materials to assess children, but there was a problem with the top-down imposition of test targets.
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