Five leaders of the progressive Korea Teachers and Education Worker's Union (KTU) were summoned to the Seoul Central Public Prosecutors' Office Wednesday over the group's anti-government statement issued twice earlier this year.
The investigative body said the unionists broke the Civil Servant Law that prohibits them from engaging in political activities and taking collective action.
But the teachers refuted this, saying they have the right to make individual political statements as citizens.
The prosecution has secured a list of 89 unionists against whom the education ministry has filed a suit. Those responsible for the anti-government statements are expected to be indicted.
The education authorities have already decided to dismiss or suspend 88 of them. Regional educational offices also decided to discipline another 79.
Jeong Jin-hoo, the head of the unionists, said that the prosecutors questioned their purpose in issuing statements against the administration and whether they were politically motivated.
He said the summoned exercised their right to silence during interrogation.
Jeong said the statements were inspired by displays of defiance made by other citizens ranging from university professors and artists to Internet users.
The leader also claimed that the prosecutors asked irrelevant questions such as ones about next year's regional elections and other civic groups.
He noted the investigation has also been tracking bank account records of the leaders. "Korea is heading back to the authoritarian era where citizens are summoned for expressing what they believe in," he said.
A spokesman of the teachers' union reportedly said they are willing to go on trial.
A total of 28,000 teachers signed the statements in June and July, asking the Lee Myung-bak administration to apologize for policies favoring the rich.