Classroom indiscipline is the biggest cause of stress to Welsh teachers, with more than one in 10 reporting physical assaults by pupils in the past year.
Poor pupil behaviour is also the most significant contributor to workload pressures, closely followed by pupil assessment requirements, say NASUWT Cymru members surveyed by the union.
Other key education reforms in Wales - including the Welsh baccalaureate and 14-19 changes - are also adding to teacher workload.
The union is warning that teachers will need more time than the guaranteed 10 per cent for planning, preparation and assessment to deliver the Assembly government's education agenda.
More than 2,160 NASUWT Cymru members responded to the survey, undertaken in September. Sixty-three per cent said pupil behaviour had adversely affected their workload, with 68 per cent saying it was a key stress factor. More than three in five cited verbal abuse from a pupil, and 11 per cent reported a physical assault.
Where schools had implemented workload agreement reforms, respondents said their work-life balance had improved. But the survey found some teachers were still doing bulk photocopying and recording results on spreadsheets.
And key changes in Wales, including the abolition of key stage tests, 14-19 reforms and the Welsh baccalaureate, are adding to teacher workload.
More than two-thirds of primary teachers said their schools had used this summer's optional KS2 Sats papers. But more than a third had ended up marking them themselves, even though external marking was available. Nearly half said their school's assessment plans involved more marking.
The report concludes that national moves towards more teacher-led assessment "are placing significant demands on primary and secondary schools".
A third of secondary teachers said 14-19 reforms had also added to workload, and some also cited the Welsh baccalaureate as burdensome.
Most of teachers' excessive workload is generated at school level, the survey suggests, although respondents were also keen to see less externally-generated red tape. It says heads have a key role in carrying out workload reforms and acknowledges that many have seen their work-life balance worsen.
But it also found some heads still resisting implementation of the workload agreement, often for funding reasons, nearly three years after it was signed.
Geraint Davies, the NASUWT Cymru secretary, said the survey showed that education reforms were in danger of overloading the profession. "We have had so many proposals, changes and reports, and all this tends to deflect from the core activity of teaching and learning," he said.
Education and lifelong learning minister Jane Davidson welcomed the finding that reforms had improved teachers' work-life balance. But she said criticisms of teacher assessment were hard to accept as tests had been abolished and teacher assessment requirements remained unchanged. She noted only 17 schools are piloting the baccalaureate.