NASUWT general secretary Chris Keates said while the conditions teachers enjoy in Scotland had been the "gold standard" in the UK, they are now coming under "increasing and damaging pressure".
She was speaking ahead of the union's Scotland conference starting in Edinburgh tomorrow, with issues including teachers' pay, education budgets and teaching reforms all due to be debated.
One motion submitted ahead of the two-day conference brands the "attacks on teachers' pay and conditions of service" as "attacks on children and young people".
Union members will also debate the "cuts to education budgets taking place across the country", which one motion says will "undoubtedly have a negative impact on learners and on staff in schools".
It goes on to suggest the union should press the Scottish Government to "insist on the ring-fencing of education budgets to prevent further cuts and the consequent damage they will cause to the Scottish education system".
Another motion condemns the "shambolic implementation of the National Qualifications" - new exams which are replacing standard grades - by the Scottish Qualifications Authority (SQA).
This adds that the "under-resourced and under pressure" exams body has "badly let down the teaching profession of Scotland with careless and complex bureaucracy to the possible detriment of candidates across Scotland".
Ms Keates said: "Teachers from across Scotland are gathering at a time when the education service, much admired across the UK, and their conditions of service, once considered the gold standard, are under increasing and damaging pressure.
"The excessive workload of teachers, combined with increased stress, relentless pressure and low morale is a dangerous cocktail. It is no surprise that many of the motions to be debated at the NASUWT conference reflect the deep concern of teachers about the changes which are being made."
She added: "Teachers are committed to doing their best for the children and young people they teach, but to do this they need working conditions which enable them to focus on teaching and learning. They also need adequate resources and support in order to meet the demands of curriculum and qualification reform.
"Teachers at the conference will be calling on ministers to listen to the serious concerns of the profession, if Scotland is to maintain its world-class schools."
Jane Peckham, NASUWT Scotland organiser, said: "The conference will provide the opportunity for teachers from across Scotland to share their concerns. They are frustrated by the fact that their professionalism and children's education is being undermined by the introduction of under-resourced initiatives and the failure to recognise the increasing bureaucratic and workload burdens they face.
"Coupled with the attacks on their pay and conditions it is unsurprising that teacher morale is at an all-time low."
A Scottish Government spokesman said: "The Scottish Government takes support for teachers very seriously and continues to work with teachers' representatives and local authorities to address any points, including the workload of those at the front line delivering lasting improvements, that will benefit our young people and economy for years to come.
"We recognise the excellent work in schools across the country and are committed to further improving our education system, as shown by the unprecedented package of support and resources provided to implement Curriculum for Excellence. This included the announcement, in February, of an additional £5 million package of support for local authorities to support teachers and schools.
"The NASUWT has supported the work that the Scottish Government is undertaking to tackle bureaucracy. We are continuing to work with the NASUWT and other partners to reduce unnecessary paperwork in order to free teachers to concentrate on what they do best, the delivery of teaching and learning."