Teachers have warned they will resist any attempt to shorten the six-week school summer holiday.
A move to cut the traditional summer break would harm children's learning and teachers' wellbeing, the National Union of Teachers (NUT) said.
The union passed a resolution at its annual conference in Torquay on Tuesday, warning it will resist any national or local attempts to impose changes to the school year, or day, against the professional judgment of teachers.
It comes weeks after NUT members in Nottingham staged a one-day walkout over proposals by the city council to introduce a five-term school year from 2013 and cut the summer holiday from six to four weeks. There are plans for two more one-day strikes later this month.
Nottingham City Council says the move will help boost attainment and attendance in its schools, arguing that many children "fall out of the habit" of learning during the long holidays. It said having shorter breaks means a greater chance that pupils will remember what they have learnt.
Speaking during the debate, Sheena Wheatley, a teacher from Nottingham, said: "A five-term year represents a major attack on our conditions of service."
She added: "I don't think I need to describe the impact of shortening the summer break, not just for us and our families, but also for the young people that we work with.
"The projected eight-week terms would have a major effect on our workload and ultimately our health, I believe."
Fellow Nottingham teacher John Illingworth said that if the plans went ahead, the city would have "the shortest school summer break in the world, at just over four weeks".
"It's true that South Korea have only five weeks, and a much longer working day," he said. "They also have the highest child suicide rate in the world."
Mr Illingworth said: "We don't want Nottingham to become a laboratory for testing how far we can drive our young people."
A six-week break is important for children and teachers, Mr Illingworth said.
"This union has stood up over the years for the right for children to play. There's been an attack on play in our schools and we're attacking children's right to play outside school."
Tom Unterrainer, also a teacher from Nottingham, told delegates: "We've looked for rigorous academic research which points to the fact that learning loss takes place. There is none. But there's plenty of evidence, plenty of empirical evidence, real world evidence out there, around the world, to show that school holiday length and the lengths of teachers' holidays and the length of time that students and young people are out of school has no verifiable impact on their outcomes. It's a nonsense."
The call to resist changes to the school holidays was part of a wider motion on teacher workload which warned that the amount of work teachers face is getting worse.
It called for the union to campaign against the problem, including national ballots for strike and non-strike action.