Job loses due to budget cuts or falling rolls will be met by strike action, the National Union of Teachers agreed.
Falling rolls should lead to smaller classes and guaranteed non-contact time, with teachers providing cover, delegates said.
Maureen Shevington, a member of the NUT executive, said 250 redundancies were threatened in the north-east region. She said: "We warned ministers last year that because schools were having to use their balances, there would be no scope to use them again."
A report for the union by consultant John Atkins found schools were most vulnerable if they had falling rolls, did not receive extra cash for schools in challenging circumstances, and had lots of experienced teachers.
By September 2005, all teachers should have at least 10 per cent of the school week free for planning, preparation and assessment under the workforce agreement. The study found primaries had made little progress towards giving guaranteed non-contact time to teachers, although this is widely available in secondary schools.
Schools were improving teachers' worklife balance but often at the expense of the head, whose workload increased.
Tim Yeo, shadow education secretary, pledged that a Conservative governmnent would "surpass" Labour's spending.
Conference gave virtually unanimous support for action against a multi-year pay deal, alongside a claim for a 10 per cent pay rise with progression up main and upper pay scales. Further action against the Government's proposal to increase the pension age for to 65 was also backed.