I feel a little late to this post, but I hope it's not too late to dispell some rumors or misconceptions. Koh Tao is a beautiful island with a fantastic vibe and community feel. Under the right conditions the dive sites are great, not world class, but perfect for teaching. Among other things, that's why Koh Tao is such a popular scuba mecca for new divers. It's on the backpacker trail, it's fun, it's cheap, and the water conditions and dive sites are perfect for learning to dive. This also means that Koh Tao is perfect for dive masters and new instructors to experience novice divers and teaching initial scuba courses. The influx of new divers and non divers gives new dive professionals chance to get valuable experience in a safe environment, with experienced instructors, with customers who are normally laid back and easy going.
Koh Tao has a large number of dive centers all targeting a different market. Some are party schools, some are cheaper than others, some are more isolated, and of course there's the decision between PADI and SSI.
For anyone looking to do a quality, not quantity, focused course, be that entry level or professional, I'd thoroughly recommend Master Divers. They're located in Mae Haad and so are a comfortable distance from the tourist hotspot Siaree (yet still close enough for the nightlife). The whole operation is amazingly well run by enthusiastic, friendly and experienced staff that pride themselves on delivering quality and thorough courses. The course director Gaz personally teaches all Divemaster programmes and runs all the IDC's. There's an emphasis on safety, standards and professionalism and after finishing my DM and IDC there I can promise nowhere else will make you into a better, more confident and employable dive professional (All while having a great time with an amazing bunch of people!). Master Divers are well known for their quality and standards around the island, and being a smaller school you get much more personal attention and better integrated into the dive team than I'd expect at some other larger factory schools.
Good luck to anyone pursuing dive training in Koh Tao!