NASUWT teachers at a school near Birmingham start a series of 24-hour strikes today as part of a campaign to try and stop it converting to an academy.
Redhill school in Stourbridge has been closed for the day by its governors and head teacher Stephen Dunster who voted for the academy route in November.
NASUWT said its members believed a conversion would be a backward step for the school which recently recorded some of its best ever GCSE results.
Teachers also worry it would lead to a decline in educational standards as well as an "erosion of vital and hard-fought-for safeguards" for teachers.
The union's general secretary Chris Keates said: "There is no educational or financial imperative to justify converting Redhill School to an academy."
She said the proposal to convert it to academy status had already been considered and rejected in 2010.
"The governing body should cease this obsessive pursuit of academy status which parents, staff and the local authority oppose."
In a message to parents on its website the school said a working group has been set up comprising governors and staff to look at conversion details and governors will make a final decision in "due course."