Go to labor court
In response to Isabella's letter ("How to take legal action", Postbox 19th April) unless it's Bangkok, take a Thai translator with you and visit the nearest labour court. Chiang Mai is "Region 5". Bring your documents and file a lawsuit. It will start out as an arbitration hearing but in case you need to pursue this further, there are Thai lawyers who speak excellent English and don't have the wildly overpriced fees of the big law firms. We are talking some 20-25k for the whole lawsuit. Many schools like to get new staff. Often it's just for financial reasons. Looking back at schools I worked at, I noticed lots of Asian and African replacementd at lower salaries than previous teachers. Make them fire you! This will trigger a notice period and severance - a few more months' at least. To resign would be plain stupid. Good luck!
Kru Anon