Headteachers have created a classroom culture where teachers compete against their colleagues to show they are working hard, according to new research.
Teachers often spend valuable time meeting external monitoring requirements, such as preparing elaborate lesson plans, in the hope of pleasing inspectors.
John Bangs, head of education at the National Union of Teachers, said the findings showed that cutting workload in schools requires a change of culture.
The research, commissioned by the union, is based on visits to 30 schools by consultants John Atkins, David Carter and Mike Nichol who looked at the impact of class sizes, preparation and assessment on teacher workload.
They found that while smaller class sizes meant less time spent on behaviour management, it did not necessarily lead to an overall reduction in the hours worked.
"Teachers tend to feel that having fewer pupils in a class means that they should spend longer over the marking of each one," they said.
One teacher said it was much easier to write up lesson plans after the lesson because it was clearer what had been accomplished.