Ministers received a clear signal of trouble ahead this week as the Profesional Association of Teachers expressed grave misgivings over the school workforce deal.
Members of the association are not prone to the fiery bouts of rhetoric that delegates from other teaching unions often indulge in. They stand up when ministers enter the room, and they never take industrial action. So the PAT's warning that giving teachers half a day a week from September for planning, preparation and assessment (PPA) time could damage primary standards is yet another indication of the strain the deal is under.
The National Association of Head Teachers has already pulled out of the agreement because of concerns about funding, particularly in primary schools.
Kathleen Barraclough, head of St Anne's Roman Catholic primary in Banstead, Surrey, told the PAT's annual conference in Buxton, Derbyshire, that PPA provision was being led by the needs of budgets rather than the curriculum or pupils.
"We do not want to see standards compromised," she said.
She said it was rare for schools in her area to have the money to provide PPA cover with teachers, and that in one school there was a plan to double up junior classes for one afternoon a week.
But Jacqui Smith, the school standards minister, told delegates that the 10 per cent non-contact time was "a must, not an option", and she later revealed that 99 per cent of schools had a plan to implement it.
When asked about doubled-up classes , she said heads should do what they thought was right for their schools.
Ms Smith was also careful to stress the key role of support staff, a canny move to a union in which a quarter of members are support staff or nursery nurses and just 63 per cent are serving qualified teachers.
Elsewhere, delegates also called for an end to national tests for 11-year-olds.
Geraldine Everett of Hazel primary, in Leicester, said: "Our pupils are used as political footballs in a testing target game where the goal-posts are moved year on year."
Delegates also voted for the Government to review medical procedures in schools and ensure they are carried out by trained medical staff.
Rosemary Stokes, assistant head of Ash Field special school, Leicester, said teaching assistants were often being expected to carry out difficult procedures such as catheterisation.
Around 100 delegates attended the conference. Other motions included a condemnation of the suggested A** grade at A-level, a warning that specialist schools would result in less choice, and a call for the reintroduction of grammar schools.
It also emerged that the association's controversial proposal to replace the word failure with "deferred success", due to be discussed yesterday, had led to a bout of hate mail. One e-mail from Australia called for its proposers to be shot.