What are the regulations for work permits and non-immigrant B visas in Thailand when changing schools?

If you have the Work Permit, take a look inside. There is your name, job prescription and place of work all in English. Yes, the work permit is only valid with the Employer who helped you to apply for this, it can not be changed to other employer, you cancel this one and with help of new employer you apply for new one. There is also a valid until date in the work permit. Extensions of Stay are also motivated and depending on your Employer, you can not switch them around. If you still inside Thailand on your original Visa and the visa is not marked with your Employers name (handwritten text or sticker next to the visa) you don't have to leave Thailand to get a new visa if you switch employers. If you visa is marked, you have start all over again and face the in your eyes unnecessary hassle.