Ask ten different teachers and you'll hear ten different experiences. Some teachers will go to local clinics, pay about 50 baht for the medical check, be in and out in ten minutes and the doctor won't even pick up a stethoscope in anger.
Others will go to a proper hospital, pay anything up to 700 baht and actually be required to give a blood sample and answer a few questions. Conclusion? It all depends where you go. But the end result - obtaining the medical certificate - is the same. Getting a medical certificate is not a big deal at all.