Schools' Minister Jacqui Smith has defended workload reforms for teachers as key to raising standards in schools.
Jacqui Smith told teachers workload reduction plans had to go ahead
In a speech to delegates from the Professional Association of Teachers, she said an agreement to allow planning time for teachers must be implemented.
But some delegates said this could mean one teacher in charge of two classes - effectively doubling class sizes.
They are concerned that the agreement could mean classroom assistants taking on a wider role.
Delegates at the PAT's annual conference voted in favour of a motion warning that extra preparation time for teachers could be detrimental to primary school lessons in particular.
But School's minister Jacqui Smith said effective support staff allowed teachers to focus on the needs of every child.
Delegates also voted in favour of scrapping tests for 11-year-olds because they say they put too much pressure on primary school pupils. The first formal tests should be at the age of 14, they agreed.
Geraldine Everett, from Hazel Primary School in Leicester, who proposed the motion, said pupils were "tested to destruction" and said the results were becoming increasingly meaningless because of political interference in the standards and basis of the tests.
Reform 'a must'
From September teachers are entitled to spend 10% of their working week preparing lessons and marking work.
Schools' Minister Jacqui Smith said implementing the preparation, planning and assessment time agreement (PPA) "is a must, not an option".
She praised the "excellent work" of support staff, which includes teaching assistants and learning mentors, who offered "tailored support" to pupils, and criticised those who denigrated their contribution.
The newly-elected chairman of the teachers' organisation, Lynn Edwards, said in a speech to the conference that PAT supported the planning, preparation and assessment time deal, calling it "very welcome and long overdue".
But she said: "My concern is that we might end up with classes being dealt with by people who are unqualified."
Kathleen Barraclough, head of St Anne's Catholic Primary School in Banstead, Surrey, who put forward the motion, said for some schools the only affordable option was putting an unqualified assistant in front of the class.
She said extra planning time for teachers was vital - but that the agreement had to be funded properly.
It is expected that primary schools will find the deal most difficult to implement because they have fewer staff to step in to cover lessons.
Delegates warned heads were facing drastic options such as cutting staff to fund the extra cover or doubling class sizes, with one teacher in charge with one or two assistants.
Head teachers and unions are concerned they will not have the funds available for extra teaching cover, and some heads have said they may be forced to break the agreement.
Speaking after the conference, Jacqui Smith said: "What is important is that head teachers make the decisions they think are right for their schools in the context of the extra funding we have put into the system."
A spokesman for the Department for Education and Skills said a majority of schools are already implementing the workforce agreement.
"We are working with schools to ensure the funding settlement for 2005/6 is designed to meet the costs of implementing PPA and this includes a minimum funding guarantee of 5% per pupil for primary schools."
David Cameron, Shadow Secretary of State for Education, described the government's position as "extraordinary".
"If the only way schools can meet the requirements of the agreement is by increasing class sizes so radically, something is clearly very wrong indeed.
"If ministers continue to dismiss genuine concerns, the potential benefits of this agreement will be completely undermined."
David Hart foresees trouble implementing the workload deal
The National Association of Headteachers voted to pull out of the agreement in March.
The largest teachers' union, the National Union of Teachers, did not sign up to the deal and maintains its opposition because of concerns over the role of classroom assistants.
But the NASUWT has criticised those who oppose the deal, saying money is available in schools' budgets.
Jacqui Smith also said extended schools would provide an invaluable role in boosting learning and providing services for parents and children.
She said they would not be an added burden for schools or teaching staff.
"More autonomous and flexible schools can focus more intensely on higher school standards for all," she said.