Teachers may refuse to take classes of more than 27

Teachers warned of industrial action yesterday that could see thousands of children being sent home from school in protest at government plans to reform the profession.

Delegates at the National Union of Teachers' annual conference in Harrogate backed a call to refuse to teach oversized classes or cover the lessons of absent colleagues for more than a day.

The vote was in protest at proposals in the Government's reform package that would allow classes to be doubled up and taken by a qualified teacher with a classroom assistant and for classroom assistants to take over lessons.

The hardline stance, outlined in a left-wing amendment to an emergency motion on the Government's package, was agreed by 120,744 votes to 88,277. A final vote on the motion will take place today.

Doug McAvoy, the union's general secretary, warned that action on class sizes would lead to children being sent home. He said he would have been happier if the amendment had been lost and added that it might be unpopular with parents and teachers.

"They don't like having to identify three children on a Monday, a different three children on Wednesday and three children on Thursday and Friday who have to be sent home on a rota basis," he said. "Class-size action has never been liked by our members and there can be no expectation it will be universally welcomed by them." If today's final vote confirms the move, the union will ballot its membership on sending children home if any class has more than 27 pupils – though headteachers may be the ultimate arbiter.

On Sunday, the union voted for a boycott of national curriculum tests at ages 7, 11 and 14 and strike action over threatened redundancies because of shortfalls in school budgets.

Delegates also said the Government's proposals would lead to a "two-tiered system", with struggling inner-city schools filling teaching vacancies with classroom assistants.

Hank Roberts, an NUT executive member from Brent, north London, said: "Will it be in the leafy suburbs or better-off areas that there are classroom assistants taking lessons? I don't think so.

"Parents wouldn't have it. They want to have their sons and daughters taught by a fully qualified teacher. It is the poor schools in the poor areas that will end up having more under-prepared and unqualified staff taking classes – inferior quality staff for the factory fodder, for the shelf stackers and parents and carers who won't or can't – they hope – make a big enough fuss."

The NUT also warned that reforms for the 14 to 19 age group would result in school selection at 14. Pupils will be able to choose between academic or vocational options or a mixture of the two under the proposals. But Malcolm Horne, from Brent, argued that would lead to selection at 14.

Damian Green, the shadow Education Secretary, promised a "bonfire of targets", adding: "If a teacher misses a target, they change the teacher. If a minister misses a target, they change the target. That's not fair."