Many schools are failing to implement a deal to reduce teachers' workload five years after it came into effect, according to Elaine Edwards, the new general secretary of the teaching union UCAC.
Ms Edwards said her belief was that a lack of resources, especially in Wales, meant teachers would rather work extra hours without their entitlements than let pupils' education suffer.
Under the workforce agreement, teachers no longer have to carry out more than 20 administrative tasks, such as filing; they should only provide cover for 38 hours per year, and 10 per cent of their timetabled teaching time should be for planning and preparation.
Ms Edwards, who succeeds Gruff Hughes as the general secretary of the predominantly Welsh-medium union, said the workforce agreement had failed to make an impact, especially in Wales.
"It's just so difficult for teachers to say no to extra work," she said. "Teachers are accountable every hour of the day - to pupils, the head, parents and governors."
Ms Edwards also plans to step up the union's campaign for a Welsh federal college, a statutory body that would be responsible for developing Welsh- medium education. She wants to see lecturers in higher education institutions funded to teach in Welsh.
"At the moment you get sixth formers wanting to go to university and study in Welsh, but when they get there they're told there aren't enough people for the course of their choice, or that the person teaching it has left," she said. Ms Edwards sees sixth forms, rather than FE colleges, as the guardians of Welsh-medium education.
The union also feels Welsh- speaking students will miss out under the 14- 19 learning pathways, a vocationally led curriculum gradually being introduced by the Assembly government that will give students more subject choice.
"Although we support wider learner choice in principle, we've concerns about how much is going to be provided in English and Welsh," she said. "You can't say you're offering greater choice if you only have 10 subjects in Welsh and 25 in English."
Ms Edwards takes over UCAC from Gruff Hughes who retired last month. Reflecting on his three-year leadership of the union, Mr Hughes, who will remain as a trustee, said his aim had been to put the union on a more professional basis. "We've also tried to raise its profile, and that's been achieved to a degree, he said. "Membership has increased to around 5,000."
Mr Hughes, a former head, spent 10 years with UCAC. "Some people may have assumed we were just a small clique of friends but we've had some successful activities over the last few years - we've proved a point," he told TES Cymru.