In regard to the sign a new contract every year as a way for Thai business to avoid severence pay is not a case that holds up with a labour board. Most teachers are not willing to look into the foreign workers department at the Thai labour department to find out what they are entitled to by law. If a person works a minimum of 120 days they are entitled to 30 days severence pay. If you sign a resignation letter you will not get a severence and most Thai schools try to trick you into signing a resignation letter.
If Thai business uses the excuse of you are only on a yearly contract than by law they would have to pay severence pay every year as they are terminating contracts yet rehiring teachers that they want to be back for another year. It doesn't hold up in the labour department and most foreign teachers don't know how to pursue severence pay when it is deserved to them.
I have one friend that went through this process as his thai wife knew what he was entitled to. He was a good teacher and he was let go after 6 months by the foreign principle (a drunk with no degree) who preferred to hire no degree teachers like himself. My friend was awarded 90,000 baht and the school had to pay