The Public Service Association in Tonga says it is prepared to sue the government as well as take strike action if its members' grievances are not addressed.
The PSA has already issued a notice of strike action for next week by teachers if their concerns about appointments are not looked at.
The PSA assistant secretary, Mele 'Amanaki, says teachers who took part in the public servants' strike are being overlooked for promotion and some have been transferred to the outer islands.
Ms 'Amanaki says the discrimination is against the Memorandum of Understanding which ended the strike last year.
The MOU stated that no one would be disciplined or discriminated against over the strike.
Ms 'Amanaki says it is not just teachers who are affected.
"Last year we have submitted two letters to the cabinet reporting some grievances of discrimination that has happened in some of the ministries and they didn't respond to that. Based on the reply, if they still breach the MOU we will take it up together with the others. It's different ministries. We are prepared to take this up to court."