In a move likely to be a test case for how much strikes should cost individuals, the National Union of Teachers claims its members in Kent are up to pound;150 worse off than colleagues in other authorities.
The dispute revolves around the two days of industrial action the union took in its call for higher cost-of-living allowances in London and the south-east of England last year.
Thousands of NUT members went on a one-day strike in March. In November, the National Association of Schoolmasters Union of Women Teachers joined the second day of action.
Both the union and the Tory-run county accept that when a teacher goes on strike, they should lose one day's pay. But they are divided about how this is calculated.
The NUT argues the figure is clearly defined, in official conditions of service papers, as 1365th of a teacher's annual salary.
Kent, acting on advice from the National Employers' Organisation for School Teachers, says the figure should be 1195th, because teachers' contracts say they only work for 195 days a year.
For a teacher earning pound;20,000 a year, this is the difference between losing pound;55 against pound;103 a day. For those on two management points, the difference would be pound;71.72 .
After the first strike in March, several councils took the employers'
advice and deducted at the higher rate.
Graham Clayton, senior solicitor at the NUT, said all except Kent had now backed down, some repaying teachers the lost cash.
He said: "The law on this has been very clear for many years. It's wholly unjustified to try to change it unilaterally, as Kent is trying to do. Even if Kent believes that the law is not clear, it really ought to think about the damage it is doing in its relationship with teachers by taking this position."
Rob Semens, employee relations manager for Kent, said that advice suggested the legal position was unclear. The commonsense case was that, as teachers are contracted for 195 days a year, they should lose 1195th of a year's pay.
He said: "Our (council) members are not happy about just accepting the NUT's position because other authorities have. There is an issue of principle here."
The NUT is due to lodge papers with the High Court next week. The case is expected to be heard in the spring.