It's sad to read so many readers' posts asking for help and reliable answers to questions regading Thai law and taking employers to court over unfair dismissals, etc.. There are two courts which do help. For private employers, it's the entirely FREE Labour Court. Find yours in your "region". Chiang Mai's is in region 5.
Government schools can and should be sued in administrative courts. Pay a 2% fee based on the amount claimed. Get someone to write the complaint in Thai... Yes, it takes a while. No, you can win. By giving up on just claims, many reward such behavior, making it more difficult for all others who follow them. Be that SS contributions, lack of notice, breach of contract. Whatever.
The Labour Court has a settlement conference, pushing hard to settle there and then. And you will be provided an attorney. The only cost will be a translator (can be a friend).
Do something for the fellow chalkies suffering from injustice and fraud.