By Sean Coughlan
BBC News education reporter, at the ATL conference
Teachers are ready to take strike action in support of their right to a reduced workload.
Mary Bousted is angry at heads' attitudes
Association of Teachers and Lecturers members have condemned heads who say they cannot afford to introduce lesson preparation time.
The union says it will back teachers who want the work reduction deal to be implemented.
ATL leader Mary Bousted says heads cannot ignore the contractual rights of teachers.
The union wants to defend an agreement on reducing workload, hammered out between the government and most of the unions.
It is infuriated by a head teachers' union walking out on the deal.
The National Association of Head Teachers says there is not enough funding to implement the agreement this autumn.
But the ATL, holding its annual conference in Torquay, says heads must give teachers the extra preparation and planning time they have been promised - or else they will face strike action.
Dr Bousted has already expressed her anger, accusing some male heads of having a sexist "cock of the roost" attitude towards female staff.
Delegates are also discussing a strike threat over changes to teachers' pensions.
But the greatest attention at this week's conference will be paid to the visit on Wednesday of the Education Secretary Ruth Kelly.
After a frosty reception at a The Secondary Heads Association's conference earlier this month, the response of the ATL towards Ms Kelly will be under close scrutiny.
Dr Bousted said the reception would be "positive", but she urged the education secretary to address the interests of teachers.
Ms Kelly's key theme is likely to be addressing the concerns of parents.