Greece on Monday took emergency action -- the third this year -- to ward off a strike by secondary school teachers timed to coincide with university entry exams.
The emergency order to work follows similar steps taken against metro workers in January and seamen in February as the conservative-led coalition government fights to minimise labour unrest.
"Strikes during exams are unacceptable. Society deems them repulsive," Prime Minister Antonis Samaras said on the issue in an address to Greek industrialists.
The federation of secondary education state school teachers OLME has refused to back down, but Greece's highest administrative court on Monday rejected its request to overturn the government's order.
OLME has also called on the country's two main unions, GSEE and ADEY, to hold a general strike on Friday, the first day of the exams.
Public sector union ADEDY decided to hold a 24-hour strike in support of the teachers on Tuesday as well as joint work stoppages with private sector union GSEE on Thursday.
Hundreds of teachers protested in central Athens on Monday against the reform, some of them dressed in mock army uniforms.
"It seems we will wear military uniforms and schools will turn into barracks," said unionist Themis Kotsyfakis ahead of the demonstration.
OLME opposes expected job cuts, obligatory transfers and an extension of teaching hours.
It also objects to school closures and mergers.
But the prime minister insisted: "Union mentality cannot be allowed to blow up society because teachers were asked to work two extra hours a week."
School teachers have suffered major pay cuts as a result of the harsh austerity programme imposed on the indebted country by its international creditors.
"We have lost our salaries. All of us could lose our jobs at any given time," high school teacher Katerina told AFP during an anti-austerity protest earlier this month.
Facing a sixth year of continuous recession, Greece was obliged to adopt a strict austerity programme that includes salary and pension cuts as part of its EU-IMF bailout deal.
The European Union and the International Monetary Fund have committed a total of 240 billion euros in rescue loans to Greece since 2010.