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.

"What kind of teachers choose to go on strike at the most sensitive time for the students?" government spokesman Simos Kedikoglou told To Vima radio.

"It is base cunning to decide to go on strike two days before the exams. They could have planned their protest before the Easter holidays," he added.

Refusing to back down, the federation of secondary education state school teachers OLME made a formal appeal on Monday to Greece's highest administrative court 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 May 17, the first day of the exams.

Public sector union ADEDY decided to hold a 24-hour strike in support of the teachers Tuesday as well as joint work stoppages with private sector union GSEE on Thursday.

OLME opposes expected job cuts, obligatory transfers and an extension of teaching hours.

It also objects to school closures and mergers.

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.