Filing a complaint for unfair dismissal

I was recently given notice by my school under the order of the headmaster there that I was no longer needed, and to start looking for another teaching job with no explanation as to why and with no written warning process implemented. I have worked there for 2 years. They offered me no severance pay. They are trying to make me sign the work permit cancellation form which they originally dated under the Date of Resignation section as March. I have chosen not to sign it at this point until the date is amended to April 30, the date my work permit expires, along with 90 days severance and a termination letter which I need to process at immigration to cancel my visa and 90 day extension. Im in the process of claiming 90 days severance which I understand Im legally entitled to under the Labor Protection Act (19ninety-eight) (the LPA), section 118 for termination without cause.
Since my request to receive 90 days severance they have asked that I go to the school to speak with their office staff. I believe they may try to pressure me or even bully me, or even wriggle out of it by trying to offer my job back, which I could not possibly accept after feeling so dejected for being fired by them. Is it best for me to refuse this request to go and meet them?
Assuming that the school refuses to pay severance, whats the correct procedure to file a complaint to the Thai labor department? I understand I have 60 days to file it.
Thank you.