Strikes are expected at about 50 schools over the next few months as anger grows over changes to senior teachers' pay which will cost staff up to Pounds 10,500 a year.
Teachers at Boston Spa and Crashaw secondaries in Leeds were expected to walk out yesterday in protest at salary cuts for members who hold management allowances.
The secondaries brought the tally of schools where members of the National Union of Teachers have taken strike action over the replacement of the allowances with teaching and learning responsibility payments, to six.
The NUT has held formal strike ballots at more than 30 schools and there are about another 250 where its members have indicated they favour action.
John Dixon, NUT assistant secretary, said the union hoped to resolve the disputes without industrial action but admitted that strikes were likely in a significant number of schools.
The changes were agreed between the Government and the social partnership unions - the NASUWT, Association of Teachers and Lecturers and the Association of School and College Leaders - but without the backing of the NUT or the National Association of Head Teachers.
All schools should have completed a review of their staffing structures, detailing who will get TLRs. Unlike management allowances, currently worth pound;1,163-pound;10,572 a year, TLRs are only awarded for raising standards in the classroom and cannot be given for administrative tasks.
The NUT believes thousands of teachers with responsibility for pastoral care could lose money as a result of the changes. It is pressing for schools to award TLRs to all those who currently receive allowances.
Teachers at a seventh school, Todmorden high in Calderdale, are expected to strike next week in protest at the changes.
A series of further strikes at nearby Hipperholme and Lightcliffe school in Halifax, one of the four where teachers took action before Christmas, has been postponed after talks raised hope of a compromise.