Concerns over the curriculum and plunging classroom morale are contributing to "chaos" across England's schools, a poll of teachers has found.
The survey of around 9,000 primary and secondary school staff in England showed fears over class sizes and fewer resources available to teach, at a time when many reported increasing workloads.
The results of the National Union of Teachers' (NUT) poll - taken over two days last week - found 86% of primary school staff said their morale had declined in the last two years - with 75% saying their morale was low or very low. Some 72% of secondary teachers described their morale as low or very low.
Almost half (48%) of primary teachers polled said they are considering leaving the profession within the next two years, while 65% of secondary staff said they were frustrated that a lack of services was having a negative impact on the achievement of students with special educational needs (SEN).
An overwhelming majority (85%) of secondary school respondents said Government accountability measures - such as tests designed to reflect each school's progress, rather than the achievements of its individual pupils - were "harming the self-esteem, confidence and mental health" of their students.
Almost all (92%) said those measures were reducing the quality and time for teacher-pupil interaction, "because of the way these performance indicators and metrics drive behaviour".
And there are serious concerns over Key Stage 1 and 2 tests, with 86% of primary school teachers believing the Government should cancel this summer's exam because of "the chaos surrounding implementation".
NUT general secretary Christine Blower said: "The Secretary of State (Nicky Morgan) should listen very seriously to this very grave warning, especially at a time when she is proposing in her White Paper to introduce yet another set of top-down changes."
She added: "The survey results confirm that secondary education is experiencing a multi-faceted crisis. Teacher workload is rising, while morale is falling. Cuts are biting, while the mental health of students is under increasing strain."
The survey also found three-in-four teachers said their workload has actually increased since the Government introduced special means to combat the issue.
Some 30% of primary school teachers said the shift in time necessary to do their job was "significant" since the start of the Workload Challenge, which was launched in autumn 2014 in an attempt to combat increasing and intensifying work levels across the country's classrooms.