COPENHAGEN (AP) ― Tens of thousands of Danish teachers were locked out of schools on Tuesday after negotiations with municipal authorities over weekly work hours failed to produce an agreement.

The National Teachers Union said 52,000 teachers were barred from entering schools, affecting some 875,000 pupils. Thousands of teachers held impromptu protests throughout Denmark, carrying signs and wearing white T-shirts expressing concern over a possible decline in the quality of education.

The showdown centers on who has the power to schedule teachers’ working time.

The union has refused to accept a new working agreement that is part of sweeping school reform program.

Municipal authorities want the right to determine how much time teachers spend in the classroom, and reject the Teachers Union’s demand for a cap of 25 hours per week in class, with the rest of their working hours used for class preparation.

Teachers currently spend on average 16 hours a week in the classroom.

“We are prepared to spend more time teaching in the classroom, but we believe there must be a limit,” said union executive board member Gordon Orskov Madesen.

“We also need proper time for preparation to offer quality in each and every lesson.”

Michael Ziegler, chief negotiator for Local Government Denmark, an association of municipalities, said local governments should have more say in how teachers spend their working hours.

“We believe that it should not be the Teachers Union who decides how teachers spend their working time,” said Ziegler.

“Instead, it should be decided locally and in a dialogue between the individual teacher and school management. Teachers need to have rules regarding working hours that are similar to all other employee groups,” said spokesperson. If the two sides fail to reach an agreement, the central government could step in and force teachers back into schools.