The strike, at Pontllanfraith comprehensive, in Blackwood, Caerphilly, is the first in Wales over new teaching and learning responsibility (TLR) payments, which replaced management allowances from January this year.
Teachers' union the NASUWT Cymru said nearly 20 teachers will lose money as a result of the school's review of its staffing structure, but will still be expected to do the same work.
Another group of teachers has been asked to undertake more responsibilities without extra pay, it said. But Caerphilly local education authority accused the union of adopting a hostile tone in meetings.
An LEA spokesman said it had helped arrange two meetings, and that the school's governors had spent six months consulting formally and informally on its new staffing structure. Full consideration had been given to the views of staff and unions, he said.
He added: "Unfortunately, the demands made by the union would have meant having a structure which was not in the interests of the pupils, and could have been an improper use of public funds."
But Mr Cox said consultations were a sham. "Our members would have much preferred not to strike," he said. "But they have been forced into this to protect themselves and their colleagues.
"Teachers at the school just want a fair deal, but the management wants its own way regardless of the consequences."
Yesterday, 33 members picketed the school's gates before lobbying the council's education offices in Ystrad Mynach.
The teachers will now work to rule, and further strikes have not been ruled out.