The reason is a controversial call by teachers for mother-tongue education in Turkey's classrooms - a demand seen by some as crucial to reform, but by others as threatening the whole basis of the Turkish state.
Last week saw state prosecutors call for the union, Egitim Sen, to be banned because a clause in its constitution committed its members to support the right to education in one's native language.
This is largely a reference to calls from groups within the country's large ethnic Kurdish community for the Kurdish language to be used where appropriate in state school classrooms.
Long banned in Turkey, Kurdish is the mother tongue of many in the southeast of the country. With Turkish the only medium of tuition in school, many educationalists see this as a major contributing factor to low attendance rates and poor results in many southeastern schools.
Recently, however, language laws have been relaxed elsewhere. Kurdish broadcasting has been permitted on state TV, while it is also now possible to learn the language at a private language school.
Yet in state education, it remains a highly sensitive area. The Turkish constitution says: "The Republic of Turkey's official language is Turkish" - and it was citing this article that the prosecution opened its case.
"In Turkey there is a status quo mentality that everyone must be the same," Egitim Sen General Secretary Emir Ali Simsek told Aljazeera.net.
The ethnic Kurdish minority now
enjoys greater cultural rights
"This mentality has caused great damage in social, political and economic terms. But the world is changing. Instead of trying to take these changes on board, those who have this mentality cannot stand any differences in society, despite recent reforms of the law."
The court eventually ruled that the union had 60 days to change its constitution and remove the call for mother-tongue teaching. If it fails to do so, Egitim Sen - with more than 100,000 members, the country's largest teachers union - will be banned.
This is unlikely to go down well with the European Union, from whom Turkey is hoping to get a date to start accession talks later this year.
"A prosecutor is taking a civil organisation to court for defending a basic human right such as mother-tongue education," says Hurriyet Sener, the head of the Istanbul branch of Turkey's human-rights association, IHD.
"This cannot happen in any democratic country."
Teachers have so far responded angrily to the threat of a ban. On 13 July, the day of the court hearing, many hundreds of them staged a noisy protest outside the Ankara courtroom, paralysing traffic in the city centre.
"A prosecutor is taking
a civil organisation to court for defending a basic human right
such as mother-tongue education"
head of Istanbul branch of IHD, Turkey's human-rights association
Further protests are planned for the end of the 60-day period - as the union has no intention of removing the offending clause from its constitution.
"Our attitude is clear," says Simsek. "This 60-day procedure is not appropriate for law. There has been no decision taken on the principal of the issue. This is a legal scandal. There is not going to be a change in the constitution. This is an unjust practice and we will carry this to national and international arenas."
The union has already received backing from international teachers' associations such as Education International, which sent observers to the court.
Meanwhile, behind the argument over the union lies another, wider issue for many Turks. That is - just how meaningful are the reforms the current government has pushed through, as it seeks to liberalise the country's political landscape.
The case against the union was inspired, many feel, by the powerful military, which has long seen itself as the guardian of a secular - and unitary - Turkey.
"There are some circles," says Simsek, "who are scared that if mother-tongue education happens, the country will be divided or something like that."
Turkey is trying to bring its laws
more into line with EU norms
"Once more we saw who is our real ruler," said Sener. "The army's areas of responsibility are fixed. But here we see that it is at the very centre of politics. This is still like that despite all the [EU] harmonisation laws."
The case also highlights the tremendous task facing the government as it seeks to reform laws and bring the country more into line with EU norms.
"If you change one article of the law, that may be a breakthrough, but if there are others – and interpretations of others - that contradict it, you keep running into brick walls again," says educationist Perihan Yasaroglu.
"There is a reform process under way, yet reforms really only become meaningful when they have been successfully tested in court. This is one of those cases where certain people who are against multicultural, multi-ethnic ideas are throwing a challenge at the whole way things are going."
"If you change one article of the law, that may be a breakthrough, but if there are others – and interpretations of others - that contradict it, you keep running into brick walls again"
Perihan Yasaroglu, Turkish educationist
Meanwhile, the union remains defiant.
"Turkey is a country which is multi-social, multicultural," says Simsek. "And for the cultures, language is a very important component. For a language to live and develop, mother-tongue education is very important. The sensitivity over this is really intolerance to this multiculturalism."
As the clock ticks away on the 60-day deadline towards a 15 September crunch point, there is much riding on the outcome - whatever the pedagogical pros and cons of mother-tongue teaching.