It is the third time this year that Prime Minister Antonis Samaras's government has invoked emergency law to force strikers back to work to try to show foreign lenders who bailed out Greece that the country is sticking to unpopular reforms.
The conservative-led coalition wants state high school teachers to put in two more hours of work each week and transfer 4,000 of them to remote parts of Greece in order to plug staffing gaps.
The union representing the teachers, OLME, says that under the plan, about 10,000 part-time teachers could be dismissed once their temporary contracts expire.
It has called for a 24-hour strike when exams start on 17 May and rolling strikes the following week.
Under Greek law, the government can forcibly mobilise workers in the case of civil disorder, natural disasters or health risks to the public.
The coalition, in power since last June, has made increasing use of these powers to deal with frequent anti-austerity strikes, breaking week-long walkouts earlier this year by seamen that led to food shortages on islands and metro workers that disrupted transport in the capital.
OLME plans to hold a rally in central Athens on Monday and called on the main public and private sector unions, representing about half the country's workforce, to stage a general strike in solidarity on 17 May.
Teachers will be served a civil mobilisation order to go to work on that day or face arrest.
Education Minister Constantine Arvanitopoulos said banning the strike was necessary because students had a "sacred right" to sit exams without disruption.
The main opposition has called on the government to withdraw the law and open dialogue with the teachers after the exams.
"These threats by the prime minister and his government are directly against the overwhelming majority of workers and society," the Syriza party, which opposes the country's bailout, said in a statement.
The government broke a taboo last month by agreeing to dismiss 15,000 public sector workers by the end of 2014, a key demand by its European Union and International Monetary Fund lenders to qualify for further rescue loans.