Teachers at the Clitheroe Royal Grammar School are staging a walkout over plans to convert it into an academy.
Members of the teaching unions, the NUT and NASUWT, claim academies have negative effects on the education system and endanger pay and conditions.
The local NUT also claimed a change in the school's status could signal a long term change in pay and conditions.
The school governors said academy status will yield very significant opportunities.
Kenneth Cridland, Lancashire Secretary of the NUT, claimed if Clitheroe gains academy status it will deprive other schools under Lancashire County Council of resources.
Mr Cridland added: "Why do they want to move to academy status which automatically gives governors the power to alter pay and conditions?"
A statement from the school said: "Conversion to academy school status will yield very significant opportunities for Clitheroe Royal Grammar School to be able to maintain and develop the quality of education provided for the benefit of the whole school.
"The governing body has always stated that it could not envisage making any future changes to its staff's current contracts of employment."