The country's largest teachers' union, Egitim-Sen, has survived a threat to its existence with a legal victory that has delighted human rights activists.
The case, brought by the state, was provoked by a clause in the union's constitution that calls for mother-tongue education. This is a highly sensitive subject in Turkey, where conflict over demands from the country's ethnic Kurds for education in Kurdish has been long, bitter and sometimes violent.
State prosecutors said that calling for mother-tongue education violated the constitution, which states that the country's language is Turkish and no other language may be taught as a mother tongue in any educational institution.
Alaaddin Dincer, general secretary of Egitim-Sen, said: "In terms of basic human rights, cultural rights, and for the individual to be able to protect his or her language, education in mother tongue is necessary. And the impossibility of receiving this is a contravention of rights."
The language clause had led to an earlier attempt to ban the union last year. This had also been rejected by the courts, but prosecutors had then appealed. The Supreme Appeals Court then ruled in the prosecution's favour, sparking Monday's retrial.
Mr Dincer praised the action of the court for rejecting the case against the union, saying that the rule of law had been upheld.
The case was observed by many human rights groups and European Union officials, who are looking closely at Turkey's human rights record before it starts membership accession talks in October.