Teachers at Nailsea School in North Somerset are staging a one-day strike after being asked to teach an extra hour a week from September.
The school's head teacher, David New, has been forced to make budget cuts due to falling pupil numbers, which means less government funding.
Members of the National Union of Teachers (NUT) voted to strike and have been picketing at the school gates.
Children from years eight and nine will have to stay at home.
'Workload issue'
Although the head teacher has reassured staff that they will still receive the statutory 10% planning time, the NUT is concerned about the workload with more teaching and less time to plan lessons.
Mr New said: "We are very disappointed that members of the NUT have rejected a very generous offer of mitigation for the reduction in non-contact time from four to three hours per week.
"The number of students transferring to secondary school in the catchments area has fallen by 25% over the past two years.
"This reduces the degree to which we can afford generous allowances of non-contact time, and we are still providing the recommended planning, preparation and assessment time plus additional time for curriculum and personal development."
Ann Lemon, of North Somerset NUT said: "Our members are being asked to teach extra lessons in time they would normally be using for lesson preparation or marking work.
"This means that they will have to spend more time at home working, when secondary school teachers already work, on average, a 48 hour week."
Brian Powell, a teacher at Nailsea School, speaking from the picket line, said: "We have got a great deal of support. The key emphasis is on the long term education of students and that is one of the reasons we are striking. This is a workload issue."
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