Greece's coalition government has said it will use emergency powers to prevent teachers from disrupting university entrance exams as part of their planned strike this month.
It is the third time this year that the government has used the emergency civil mobilisation order - a measure normally reserved for natural disasters and other times of national crisis - to end a labour dispute in the crisis-hit country.
A government spokesman said it took the decision under its "moral obligation" to safeguard the May 17-31 exams for school leavers and university candidates.
"We are dealing with a threat to the national interest," he said. "We cannot accept a situation that allows 100,000 children and their families to be held to ransom."
Teachers' unions had planned to start rolling strikes on May 17 to protest over longer working hours and involuntary staff transfers.
The Greek Federation of Secondary Education State School Teachers, OLME, called the decision "extreme and undemocratic" and said it would challenge the order in court. The union, which is organizing a protest in Athens and other cities said it would review its strike plans.
Civil servants' unions called a 24-hour strike for Tuesday in reaction to the government's decision to use emergency powers against the teachers' action. The nationwide strike is expected to close schools and disrupt public services. Union officials said it was also planning a work stoppage on Thursday and a series of public protests.
Prime minister Antonis Samaras signed the order after taking similar action earlier this year to end strikes by workers at ferries services and Athens' subway train system.
Pay and staff cuts have become routine in the debt-strapped country, part of the budget austerity measures demanded by international rescue lenders who have kept Greece from defaulting on its debts since it lost bond market access in 2010. The draconian cuts have seen Greece's minimum wage slashed and unemployment rise to 27%.
Eurozone finance ministers are expected to approve the payout of new bailout instalments to Greece after more austerity measures were approved last month, including provisions for the mass dismissal of civil servants.