Teachers are considering industrial action amid growing discontent about staff shortages.
Last night union members in Sussex agreed that a decision on whether to ballot staff would be made on Thursday. It could mean teachers would refuse to cover for colleagues' absence.
Paul Mundt, executive member of the National Association of Schoolmasters/Union of Women Teachers (NASUWT) for Sussex, said the move followed a decision by members in London and Doncaster to take industrial action.
He said: "We made a decision last month not to go down this line but there is now a growing momentum to take action.
"The situation in some parts of Sussex is as bad as it is in these places and members across the county are beginning to say they need to be balloted."
The National Union of Teachers (NUT) is expected to follow suit.
In West Sussex there are 42 vacancies for full-time teachers. In East Sussex the figure is 21.
Mr Mundt said: "The actual figure of teachers missing from classrooms is much higher than local education authority figures because they do not include people who are absent because of long-term illness or those posts covered by supply staff.
"There are pockets like Bognor and Littlehampton where there are enormous problems and teachers are working 50-hour weeks."
Mr Mundt said the teacher shortage was also having a detrimental effect on some children's education.
However the three education authorities which cover Sussex - Brighton and Hove City Council, East Sussex County Council and West Sussex County Council - say staff shortages are not on the same scale as elsewhere in the country.
A spokeswoman for West Sussex County Council said: "We have got some shortages in some areas, particularly in secondary schools.
"It's more difficult to recruit teachers in maths, science, music, design and technology and modern languages."
West Sussex County Council has relocation packages worth up to £4,000 to encourage teachers to take jobs in the area and is considering further incentives for newly qualified staff.
In East Sussex the number of vacancies is about average for this time of year.
A spokeswoman for Brighton and Hove City Council said: "We do recognise there is a teacher shortage but it has not been as severe in Brighton and Hove as it has been in other parts of the South-East."
National agency Timeplan, which has 140 supply teachers on its books in Sussex, is keeping a close eye on the potential industrial action.
A spokeswoman said: "We have not noticed any discernible problems at the moment but we will pull out all the stops to step in and help if it becomes a problem."
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