The National Association of Schoolmasters Union of Women Teachers, on the second day of its annual conference in Llandudno, north Wales, voted to consider balloting to disrupt schooling in areas where moves to double the current three terms go ahead next year.
Chris Keates, the union's deputy general secretary, said local education authorities were introducing the change piecemeal, risking "total confusion and chaos".
With different arrangements across the country, parents with children in schools in adjacent local authorities may face disrupted holidays and expensive childcare bills, she said.
Moves to switch from the three-term arrangement to a system with six terms of more equal length are backed by the Local Government Association, and were endorsed by 150 LEAs in November.
Supporters of the six-term year say it will even out study periods, which can drain pupils, by breaking up the autumn term with a two-week holiday and creating a fixed fortnight-long break rather than pinning the holiday to the moveable feast of Easter. It would also mean earlier exams allowing pupils to apply to university on the basis of actual, rather than predicted, grades.
The education secretary, Charles Clarke, favours the new structure, but the government is leaving it up to individual local authorities to decide whether to implement it in September 2005.
With up to 40 councils moving to introduce the change, the NASUWT argues Mr Clarke should intervene and hold a full consultation to come up with an agreed national pattern of terms and holidays. The union is preparing to hold ballots for industrial action in Essex and Hampshire on the issue. If action is approved union members will turn up for school only according to the current timetable.
Ms Keates said that in Essex, individual schools were being given the chance to amend their holiday dates further. "That is just the route to madness because the basis of the proposals for the six-term year is to equalise terms and bring some standardisation."
The union was not opposed in principle to the six-term system, but believed there were "more important things to concentrate on", she said.
If change happens, the NASUWT wants LEAs to agree on a national pattern, including the dates of any fixed spring break and limits on when the five annual training days could be taken. It also wants to retain a guaranteed six-week summer holiday, warning that any erosion would hit teacher recruitment and retention.
Delegates in Llandudno rejected a move to endorse a fixed spring holiday separate from Easter. The move, which might see a four-day return to school between the two breaks, would "cause mass absenteeism," said Graham Jackson, a delegate from Wokingham.
Graham Lane, the education chairman of the Local Government Association, branded the NASUWT "unbelievably irresponsible" for threatening industrial action. He said: "They will deprive children of teaching time when what we are trying to do is even up the education time. This is all to do with union recruitment."
All 150 local authorities could never move at once, he said, but most would shift to a six-term system. He said the current pattern of terms and holidays was not uniform across the country.
The Department for Education said: "We support the move in principle but it is right that school terms are set locally so that they can take account of local circumstances and traditions.
"We are keeping in touch with the LGA about the decisions being reached by LEAs about moving to a six-term year and encourage authorities to bring their term dates into line with one another where possible."
