A progressive teachers' union said Monday it will stage an "all-out struggle" to regain its legal status, defying a government warning of a stern punishment.

The union has refused to comply with the education ministry's order to send union staff members back to school and instead vowed to launch an "all-out struggle" against the government because a local administrative court outlawed it for having dismissed educators among its members on Thursday.

"The education ministry prepared guidelines for having full-time union officials return to schools and various other measures to oppress the union and issued them as soon as the court ruling was made as if it waited for the decision," Kim Jeong-hoon, chief of the union, said during a news conference. "This is a clear and unjust oppression (of the union)."

He said the KTU will engage in an all-out struggle against the government until its four demands for the government are met.

The four include the withdrawal of the decision to turn the KTU into an outsider union, a revision of the law on teachers' unions and legislation of a special law for the proper settlement of the Sewol ferry disaster.

The KTU also demanded President Park Geun-hye withdraw her nomination of Kim Myung-soo, who is labeled by opposition lawmakers as a pro-Japanese and ultra-right professor, as the new education minister and the government stop its move to reinstate the state publishing of history textbooks.

During the news conference, the union reaffirmed its plan to have its members collectively take an early leave on Friday in a show of anger at the government and the court decision.

The teachers will participate in a protest rally and make protest visits to the presidential office and the central government building in Seoul during their planned leave, the union added.

More than 10,000 teachers will assemble for a national meeting in Seoul on July 12 and actively express their opinions on various pending educational issues, the union said.

The education ministry, however, has warned that teachers participating in the "illegal collective action" will be severely punished."

After the press conference, KTU officials visited the Seoul Administrative Court to file an appeal against the court ruling and an injunction for suspending the ruling's efficacy for a while.

Founded in 1998, the progressive teachers' union became a legitimate organization the following year under the then liberal government of President Kim Dae-jung and has grown into the country's second-largest teachers' union. (Yonhap)